In the four seasons of 2017, phytoplankton sampling survey was conducted in the Jiajiang in Yangzhong, and the phytoplankton community structure characteristics and trends were studied. A total of 93 species of 6 phyla were identified, including:15 species of Cyanophyta, accounting for 16.13% of the total species of phytoplankton; 3 species of Cryptophyta, accounting for 3.23%; 3 species of Pyrrophyta, accounting for 3.23%; 36 species of Bacillariophyta, accounting for 38.71%; 5 species of Euglenophyta, accounting for 5.38%; 31 species of Chlorophyta, accounting for 33.33%. There are 8 dominant species of phytoplankton in the whole year, including:3 Cyanophyta species, namely microcystis, pseudoanabaena and Anabaena circinalis, 1 species of cryptophylum, namely chromonas acuta. There are 4 species of bacillariophyta, namely Microcystis granulosa, Microcystis meniere and Synedra sp. The annual average biological density of phytoplankton is 35.34×10⁴ cells/L, with Bacillariophyta and Cyanophyta accounting for 47.45% and 42.87% respectively. The annual average biomass was 0.183 5 mg/L, and Bacillariophyta (accounting for 78.42%) had the main biomass. There were significant differences in phytoplankton biomass in 4 quarters (P<0.01), highest in autumn, lowest in spring, and no significant difference in each section (P>0.05). The RDA redundancy analysis shows that the biomass density of phytoplankton is positively correlated with water temperature, TDP and so on, which is negatively correlated with Chl.a. Estimation of fishing potential shows that Jiajiang River is enough to feed more than 8 finless porpoise.