Abstract: In recent years, the water resources security and ecological impact of the middle route of the South-to-North Water Diversion Project has been widely concerned, and the sedimentation problem gradually appears in the slow flow and backwater areas of the middle route, and the suspended solids is an important source in the main canal. To solve this problem, this study carried out research on the spatial distribution and dynamics of suspended solids in the main channel of the middle route of the South-to-North Water Diversion Project in November 2020 and July 2021. The concentration and composition of total suspended solids showed obvious spatial and seasonal differences. The concentration of volatile suspended solids was higher than that of non-volatile suspended solids in the north area of the water diversion channel. The generalized additive model was used to analyze the relation between phytoplankton cell density and suspended solids composition, the result showed the concentration of volatile suspended solids is significantly related to phytoplankton cell density, meaning phytoplankton account for a large proportion of volatile suspended solids. According to phytoplankton community analysis, Aphanocapsa sp and Melosira granulata were the dominant species in November 2020; Synedraacus and Aphanocapsa pulchra were the dominant species in July 2021. The cell density in July 2021 is higher than November 2020, the Bacillariophyta, Cyanophyta, Chlorophyta accounts for the largest proportion of biovolume and cell density in the community, they all positively correlated with dissolved oxygen and water temperature. Thus, it is necessary to pay attention to the growth and reproduction of Bacillariophyta species in the main canal to prevent them from increasing the concentration of volatile suspended solids and affecting the water receiving in the northern area of the main canal