Abstract: Mesostigma is the most closely related unicellular flagellate algae to land plants, and is one of the key groups for studying the origin of land plants. However, there is no report of Mesostigma in China. Studies have shown that Mesostigma is mainly distributed in static fresh water bodies, such as ponds, reservoirs and small lakes. This study collected several strains of green algae using plankton net in ponds from Hechuan, Chongqing and in reservoirs from Jiulong River, Fujian. In order to identify the species and figure out the phylogenetic position of the green algae, morphological observations under the microscope and phylogenetic analysis by a variety of applications and softwares were accomplished in this study. Both the morphological observation results and the analysis of the phylogenetic tree based on 18S rDNA sequence showed that the green algae collected in this study were Mesostigma viride. In addition, this study also identified the existence of Mesostigma from the environmental DNA of the Three Gorges Reservoir, and the results showed that it mainly appeared in autumn, indicating that autumn may be the best season for collecting Mesostigma in China. This is the first report of Mesostigma in China, laying the foundation for future research on Mesostigma.