This paper introduces the distribution of Elysia tomentosa K. R. Jensen, 1997 in Hainan Island for the first time. E. tomentosa belongs to Family Plakobranchidae, Superorder Sacoglossa, Class Gastropoda in the Phylum Mollusca. In September 2021, the sea slug was collected in Yunlong Bay, Wenchang city, Hainan Province. We designed the primers of 16S ribosomal RNA gene sequence, histone H3 gene (H3) sequence and tufA gene to identify what species it is and the algae in its living environment by polymerase chain reaction (PCR). We detected the photosynthetic fluorescence characteristics of E. tomentosa by chlorophyll fluorescence induction curve. A series of fluorescence parameters Fv/Fm, Y(II), Y(NO) and NPQ measured by the chlorophyll fluorescence measuring instrument Dual PAM-100 (Walz, Germany) were calculated by t-test with three samples and three repeat. Through sequence alignment of 16S rRNA and the relationship between photosynthetic characteristics and evolution, a phylogenetic tree was constructed by maximum likelihood (ML) method. The comparison of 16S rRNA sequence and H3 gene sequence identified this species as E. tomentosa by molecular method. The source of the symbiotic chloroplasts of this species was Caulerpa racemosa (Fig. 3). The photosynthetic activity of E. tomentosa could maintain for about one month and the photosynthetic capacity of E. tomentosa continue to decrease (Fig. 2). The evolutionary tree showed that the evolutionary status of E. tomentosa in the some species (Fig. 4). This is another species of photosynthetic mollusk discovered by our research group in Hainan Province, which further enrich our understanding of its distribution in China and also provides new materials for studying the evolution of photosynthesis and endosymbiosis mechanism.