To understand the stability of phytoplankton community and the interspecific association of the dominant species in lakes, surveys of phytoplankton were conducted seasonally in Lake Changhu from October 2020 to July 2021. The relationships among dominant phytoplankton species were comprehensively analyzed by niche breadth index modified by Levins, niche overlap index of Pianka, variance ratio method, Chi-square test, Pearson correlation test, and Spearman correlation test. The results showed that 8 phyla, 67 genera, and 78 species of phytoplankton were identified in four seasons. There were 19 dominant species. The main phytoplankton in Lake Changhu are green algae, cyanobacteria and diatoms. Cyclotella meneghiniana and Scenedesmus quadricauda were the dominant species in all seasons, which had an obvious indication of the eutrophication state of water environment. The results showed that the water of Lake Changhu was in a state of moderate eutrophication. According to dominance index and niche width index, the 19 dominant species could be divided into four groups. The niche overlap value varied from 0.02 to 0.96, and the seasonal difference was obvious, with autumn>winter>spring>summer. The results of variance ratio showed that there was a significantly positive association between the dominant species of phytoplankton in autumn and winter, and the community was stable. There was no significantly negative association among the dominant species in spring and summer, and the community fluctuated. The results of Chi-square test, Pearson correlation test, and Spearman correlation test showed that the significant rate of association between dominant species was low, which indicated that the association between species was not tight. In winter and spring, the number of negative association species was more, indicating that the community was in the initial stage of succession. There were more positively associated species in summer and autumn, indicating that the community was in the middle stage of succession. The results of comprehensive analysis showed that the phytoplankton community in the Lake Changhu was in a stable state on the whole, but there was still a risk of cyanobacteria bloom in summer. The ecosystem is fragile, and the comprehensive ecological protection measures need to be further implemented.