[Objectives] During the 14^th Five-Year Plan period, the Pinglu Canal in Guangxi will be a magnificent project for New International Land-Sea Trade Corridor in western China, and the Maowei Sea is a critical point of the Canal. Maoling River and Qin River flow toward the Maowei Sea. Land-sourced pollutants are inputted by these two rivers. The pollutants mainly include industrial wastewater and domestic sewage, as well as pesticides and fertilizers in agricultural irrigation areas, and nutrients and organic pollutants in surrounding shrimp-breeding pond. From 2010 to 2014, the water quality of the Maowei Sea was the worst, with pollution readings often reaching level four. The main factors that exceeded the state standard were inorganic nitrogen and active phosphate. The pH of some part of Maowei Sea also exceeded the state standard. Because the outlet to sea is narrow, and too much pollutants inputted, the routine tides from Beibu Gulf cannot effectively keep the water quality of the Maowei Sea at a healthy level. The community of macrobenthos in the Maowei Sea in 2009 was investigated, and it was believed that the Maowei Sea was in a clean to lightly polluted situation. Therefore, the present study aims to assess the ecological environmental level of the Maowei Sea using intertidal macrobenthic communities, and to collect the baseline data which would support the scientific management of the Pinglu Canal during construction and operation. [Methods] Intertidal macrobenthos were collected at four sampling sites along the eastern coastline of Maowei Sea, Qinzhou City, which included Sun Yat-Sen Park, Shahuan Village, Mangrove Park and Kangwangmiao Village, in October 2020 as well as in January, April and July 2021 (Fig. 1), according to the standard protocol for marine studies (GB 12763.6-2007). One sample line perpendicular to the shoreline is set for each site, and one sample is taken in the high tide zone, two samples are taken in the middle tide zone, and one sample is taken in the low tide zone. Then all animals from all samples mixed together, which represent the community of macrobenthos at the sampling site. Samplings were conducted once every season in every sample site for one year. All sites are outside mangroves or on bare beaches. The mud samples were filtered through sieves (mesh size:0.5 mm) in the nearby tidal creeks. The macrobenthos samples in sediments were fixed in 5% formalin in the field. All specimens of macrobenthos were brought back to the laboratory. The collected macrobenthos were preserved in 75% ethanol for further examination in the laboratory. All specimens were identified by ourselves and some taxonomy experts. All specimens were weighed by electronic balance at 0.01 g accuracy. All specimens were counted and categorized to clarify the macrobenthic composition of each sampling site. Index of relative importance (IRI) was calculated to clarify dominant species, main species, common species, and rare species. IRI = (pi + wi) fi, where pi is the relative abundance of species i in the plot, wi is the proportion of the weight of species i in the plot (%), and fi is the percentage of the number of plot of species i in the total number of plots investigated (%).We defined IRI≥ 1000 as dominant species, 100 ≤ IRI < 1000 as main species, 10 ≤ IRI < 100 as common species, and IRI < 10 as rare species. Species with IRI ≥ 100 were considered as the important species in the fauna community. Simpson diversity index (D), Shannon diversity index (H?), Pielou evenness index (J) are used to scale the diversity of macrobenthos. Nonparametric Kruskal-Wallis and Mann-Whitney analyses were performed on the macrobenthos biodiversity index data using SPSS 26.0. Then diversity of macrobenthic data was analyzed and compared to assess the overall environmental conditions of the Maowei Sea. Shannon’s diversity index H? is used to assess environmental pollution, while H? < 1, heavy pollution; H? value of 1 to 2, moderate pollution; H? value of 2 to 3, mild pollution; H? > 3, clean. Meanwhile, macrozoobenthos pollution index (IMP) assesses pollution level, while IMP <﹣6, clean;﹣6 ≤ IMP < 0, light pollution; 0 ≤ IMP ≤ 4, moderate pollution; IMP > 4, serious pollution. The multivariate analysis software PRIMER 6.0 was used to analyze the disturbance degree of macrobenthos community, and the abundance and biomass curve (ABC) was constructed. When the biomass dominance curve is above the abundance curve, W is positive, which means the community is not disturbed; on the contrary; when W is negative, it means that the community is severely disturbed and the biomass curve is below the abundance curve; When the curves overlap, the W value is close to 0, indicating moderate interference. [Results] Total of 3 084 macrobenthos individuals were collected from 4 sites in the intertidal zone of the Maowei Sea. According to the classification and identification, there are 6 phyla, 8 classes, 31 families, 51 genera and 58 species of macrobenthos in the Maowei Sea intertidal zone (Table 1). The average annual animal density is highest in Shahuan Village (6 340 ind/m²); and lower in Sun yat-sen Park (1 240 ind/m²), Mangrove Park (3 204 ind/m²)and Kangwangmiao Village (1 552 ind/m²). The macrobenthos composition in each sampling site is more than 50% arthropods, followed by annelids, about 10%, and molluscs, less than 2%. Small arthropods are the main group of macrobenthos in the intertidal zone of the Maowei Sea, accounting for 41.38% of the total species and 87.13% of the total individuals. However, the macrobenthos communities in the estuaries and intertidal zones of the Beibu Gulf in the past, were generally composed of molluscs, arthropods, and annelids in turn as the dominant groups. To date (Table 2), Discapseudes mackiei, Ilyoplax formosensis, Macrophthalmus convexus, Metopograpsusquadridentatus whose IRI are more than 1000, are the dominant species, but dominant species were not evenly distributed in the four plots, and only D. mackiei appeared in each plot. 27 main species whose IRI are more than 100 but less than 1 000 are listed in Table 2, Paracleistostoma depressum, Notodasus sp. appeared in each plot, however, the others were not evenly distributed in plots. 19 common species whose IRI are more than 10 but less than 100 were found, and 8 rare species whose IRI are less than 10 were found. According to Kruskal-Wallis analysis, the Simpson diversity index (D) and Shannon diversity index (H?) values of Shahuan Village were all lower than other three sites (Table 3). The average indices of the Simpson diversity index (D) and Shannon diversity index (H?) values were 0.435 (range 0.183﹣0.704), and 1.506 (range 0.747﹣2.256) respectively, in which the levels were significantly lower than those in 2009 (P = 0.004, and P = 0.048 for D and H? respectively). The Pielou evenness index (J) value was 0.475 (range 0.223﹣0.696) which was statistically similar to that in 2009 (P = 0.109). Overall consideration of abundance and biomass curve (ABC) (Fig. 2), Shannon diversity index (H?) and macrozoobenthos pollution index (IMP), environmental quality of the Maowei Sea is in moderate pollution. [Conclusion] Our results demonstrated that the intertidal areas of Maowei Sea were dominated by arthropods, which accounted for over 41% of the total species number and over 87% of the total abundance. The dominant species (IRI > 1 000) included D.mackiei, I.formosensis, M.convexus and M.quadridentatus, but they were not evenly distributed in the four sites, and only D. mackiei appeared in each site. These biodiversity indices concluded that the Maowei Sea is suffered from moderate pollution. The currently established intertidal macrobenthic database of Maowei Sea can support the ecological construction and scientific operation of the Pinglu Canal in future.