Under the dual pressures of global climate change and anthropogenic activities, significant shifts have been observed in the succession characteristics of lake biological communities and ecosystem structures, with diatoms emerging as one of the key indicators for assessing lake environmental changes. This study focuses on Chenghai Lake, a natural closed deep-water lake, investigating seasonal patterns in water environment, diatom community structure, and biodiversity through bi-monthly stratified sampling at four open-water stations over one year. The research evaluates the driving roles of key factors such as lake water alkalinity, eutrophication levels, and water temperature. Hydrochemical results reveal that Chenghai Lake is currently a phosphorus-limited system, with water pH consistently exceeding 9 annually and peaking in winter. Chlorophyll-a (Chl-a) and dissolved oxygen (DO) exhibit seasonal dynamics synchronous with pH fluctuations. 122 diatom species belonging to 21 genera were identified. The diatom community demonstrates a seasonal adaptive strategy of functional group shift between planktonic and benthic forms, alongside high spatial homogeneity in distribution. Seasonal differences in community structure were significant: planktonic species dominated from January to May, benthic/epiphytic species prevailed from July to September, and both groups co-dominated in November. Spatially, dominant species and their relative abundances remained consistent across sampling sites. Redundancy analysis (RDA) revealed that water temperature (~31%) is the primary environmental driver of seasonal shifts in dominant diatom species, with environmental gradients including conductivity (13.7%) and nutrients (6.1%) synergistically facilitating seasonal succession. Meanwhile, seasonal variation in thermal stratification intensity of deep lakes might influence algal growth by regulating water mixing intensity. Diatom species richness decreased with increasing water depth and exhibited significant seasonal differences (with the highest diversity in summer and autumn). Spatially, diatom diversity showed no significant differences among sampling sites due to convergent aquatic environmental conditions. The spatiotemporal variation in diatom diversity was significantly influenced by interactive effects of multiple environmental factors including temperature, water depth, and nutrients. This study provides targeted scientific insights for ecosystem evaluation and biodiversity conservation in closed lakes, offering data support for the effective protection of low-latitude plateau deep-water lakes under future climate change scenarios.