Here we introduce a new distribution location of Elysia leucolegnote in Hainan province, China. We have identified this species by its external characters and molecular approaches, recorded its behavior in its habitat location. We used the targeted PCR method to amplify the cytochrome oxidase subunit I (COI) gene. Based on the sequence of COI, a phylogenetic tree of the Elysia genus species reported in China has been constructed. This sea slug has a body length from 5﹣13 mm, the body size increases from May to November. Small herds of E. leucolegnote with 1 to 100 individuals can be easily found in the puddles of the mangrove swamp at low tide or in the crab holes for shading when these were exposed to air (Fig. 1); The external characters of E. leucolegnote are identical to the original reports, the adult sea slug is green in color with white-edged parapodia and white spots (Fig. 2); The sequence of COI gene of the sea slugs collected in Hainan conformed this species is E. leucolegnote. We uploaded the sequence of cytochrome oxidase subunit I (COI) of mitochondrial DNA (GenBank accession number MT933184). And in phylogenetic tree (neighbor-joining, NJ) 100% degree of confidence of branch shows our sea slugs were E. leucolegnote (Fig. 3). We suggest that E. leucolegnote may be potentially used as a new model system for studying chloroplast endocytosis and endosymbiosis. Therefore, this is also a unique system that may be employed in gene expression and regulation in chloroplasts. Furthermore, this model system may also be utilized in research studies on animal cell immunity.