Under the dual pressures of global climate change and anthropogenic activities, significant shifts have occurred in the succession patterns of lake biological communities and ecosystem structures. Diatoms have emerged as key indicators for assessing lacustrine environmental changes. This study examines Lake Chenghai, a natural closed deep-water lake, investigating seasonal variations in water environmental parameters, diatom community structure, and biodiversity through bimonthly stratified sampling at four open-water stations over one year. The research evaluates the driving effects of key factors including alkalinity, eutrophication status, and water temperature. Hydrochemical analyses indicate that Lake Chenghai is currently a phosphorus-limited system, with water pH consistently exceeding 9.0 annually and reaching maximum values in winter. Chlorophyll-a (Chl.a) and dissolved oxygen (DO) exhibit seasonal dynamics synchronous with pH variations. A total of 122 diatom species from 21 genera were identified. The diatom community demonstrates a seasonal adaptive strategy involving functional group shifts between planktonic and benthic forms, alongside high spatial distribution homogeneity. Seasonal variations 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) indicated that water temperature (~31%) is the primary environmental driver of seasonal variation in dominant diatom species, with additional contributions from conductivity (13.7%) and nutrients (6.1%) facilitating seasonal succession. Seasonal variation in thermal stratification intensity may further influence algal growth by regulating vertical mixing. Diatom species richness decreased with increasing water depth and showed significant seasonal variation, with the highest diversity occurring in summer and autumn. Spatially, diatom diversity exhibited no significant differences among sites due to homogeneous aquatic environmental conditions. The spatiotemporal patterns of diatom diversity were significantly influenced by the interactive effects of multiple environmental factors including temperature, water depth, and nutrients. This study provides scientific insights for ecosystem assessment and biodiversity conservation in closed lakes, offering data support for the protection of low-latitude plateau deep-water lakes under future climate change scenarios.