Yitong River is an important river in Chang-chun City. With the economic and social development of Yitong River Basin, the river eutrophication is serious and the water quality is deteriorating. To explore the seasonal variation characteristics and influencing factors of phytoplankton community structure in Changchun section of Yitong River, the water environment and phytoplankton community structure of 8 research sites along the Yitong River in Changchun was investigated in May (spring), July (summer), September (Autumn) and November (winter) in 2019. The results showed that the Chlorophyta dominated phytoplankton species in spring, summer and autumn, while Bacillariophyta dominated the phytoplankton species in winter, cyanobacteria dominated the phytoplankton density in summer and autumn, and Bacillariophyta dominated the phytoplankton density in spring and autumn. The phytoplankton density in summer was more than 2 times higher than that in the other three seasons, and the diversity, richness and evenness were in the order of spring and summer > autumn and winter. The dominant degree of Cyclotella sp. was obvious in spring (Y = 0.17), Pseudoanabaena sp. in summer was Y = 0.22), Merismopedia minima (Y = 0.48) and Chlamydomonas globosa ( Y = 0.45) in autumn, and that of Cyclotella sp. in winter (Y = 0.65). The average density of phytoplankton and the density of Cyanophyta in spring, summer and autumn showed a trend; urban area > upstream area > wetland area, and it showed that the phyto-plankton species number in Changchun section of Yitong River was in the order of upstream area > urban area > wetland area in winter. In summer, the phytoplankton density and the average density of Cyanophyta were the highest in the four seasons, which were 7.32 times and 4.38 times higher than those in the upstream area, respectively. The diversity index, richness index and evenness index of each region showed a trend of wet area > upstream area > urban area. Redundancy Analysis (RDA) results showed that the main factors affecting phytoplankton community structure in Changchun section of Yitong River were total nitrogen (TN), water temperature (WT) and chemical oxygen demand (CODCr). The urban area was mainly affected by TN during the spring and winter period, while in summer and autumn, the urban area was mainly affected by WT and CODCr. In spring and autumn, the correlation between phytoplankton and various environmental factors was relatively low in upstream area and wetland area. In summer and winter, the distribution of phytoplankton in the upstream area was scattered and the influencing factors were complex. The phytoplankton in the wet area during summer was mainly affected by suspended solid (SS), and the phytoplankton in the wet area during winter was mainly affected by TN.
