To analyze the species composition, standing stock, spatial distribution and historical change of unionids, two investigations were carried out at 129 sites in eight areas of Lake Taihu in October 2016 and October 2017. A total of 704 unionid individuals were collected, belonging to 8 genera and 14 species. The mean biomass and density of unionids were 4.169±9.337 g/m² and 0.164±0.386 ind./m², respectively. The mean biomass and density differed significantly among the areas in Lake Taihu. The biomass and density were highest in eastern lakeshore area, with the values of 14.975±16.743 g/m² and 0.577 ±0.758 ind./m², respectively. The lowest biomass and density were recorded in central area of Lake Taihu with the values of 0.727±1.622 g/m² and 0.029±0.071 ind./m², respectively. The Arconaia lanceolata, Unio douglasiae and Anodonta woodiana woodiana were dominant species in Lake Taihu. The eight areas of Lake Taihu were classified into three groups based on cluster analysis using mean density of unionids. Compared to historical data, the unionid resources appeared an obvious decline, which suggested an effective management and conservation of unionids in Lake Taihu be strengthened.