Abstract: In order to investigate the spatiotemporal pattern of phytoplankton communities and the determining factors in the middle of the Huaihe River, phytoplankton and environmental factors in 28 sections of the mid-stream river were collected and analyzed in June 2019 (flat-water period), September (high -water period) and February 2020 (dry-water period). The results showed that phytoplankton species were detected as 8 Phylum, 128 species in the middle of the Huaihe River. The densities of phytoplankton were 0.06×106~16.04×106, 0.47×106~41.62×106 and 0.43×106~10.79×106 cell·L-1 in the three water periods. The biomass of phytoplankton was 0.03~3.54, 0.12~14.99 and 0.14~25.48 mg·L-1. Microcystis, Microcystis, Cyclotella meneghiniana were main dominant species in three water period. In spatially, density of up-gate presented significantly higher than mid-gate, biomass of up-gate presented significantly higher than under-gate. Total phosphorus, temperature and permanganate index were significantly positively correlated with phytoplankton density through Pearson correlation analysis. Temperature was the main environmental factor correlate with phytoplankton community structures through CCA analysis.