Phytoplankton, as primary producers in aquatic ecosystems, reflects the ecological structure and functional status of water bodies. The water circulation structure of arid lakes is highly variable, making phytoplankton communities particularly sensitive to environmental changes. This study explored the response and successional mechanism of phytoplankton communities in Lake Ulungur to seasonal environmental changes, examining factors such as species composition, diversity, dominant species, and interspecific interactions across different seasons. The surveys were conducted in May (spring), July (summer), and October (autumn) of 2019. A total of 161 phytoplankton species from 73 genera and 7 phyla were identified across 28 sampling sites, with 85, 49, and 149 species detected in spring, summer, and autumn, respectively. Tetraëdron minimum was the dominant species in all three seasons. The absolute dominant species in spring were Oocystis lacustis and Synedra acus, while Aphanizomenon flos-aquae and O. marssonii dominated in summer, and Planktolyngbya circumcreta and Aphanizomenon flos-aquae dominated in autumn. The interspecies association characteristics showed significant positive associations among dominant species in spring and autumn, while significant negative associations were observed in summer. Phytoplankton diversity was highest in spring, followed by autumn and summer. Non-metric multidimensional scaling and permutational multivariate analysis of variance indicated significant differences in phytoplankton community composition across the three seasons. Multiple regression on distance matrices analysis revealed that water temperature, transparency, and nitrate nitrogen positively influenced community heterogeneity, while the permanganate index had a negative effect. Additionally, the community structure was closely related to the abundance of Oocystis marssonii, Tetraëdron minimum, Microcystis sp., and Staurastrum sp. Overall, the phytoplankton community in Lake Ulungur exhibited stable succession patterns under the influence of seasonal environmental changes and interspecies interactions. This study provides essential ecological insights into the response of phytoplankton in arid lakes to environmental factors and interspecies relationships.