Chenghai Lake, a significant water body on the Yunnan-Guizhou Plateau, plays a pivotal role in the improvement of its watershed’s ecological environment. This is not only essential for Yunnan’s achievement of the three major ecological security tasks but also serves as the foundation for sustainable socio-economic development. In recent years, Chenghai"s water environment quality has faced serious challenges due to both human activities and natural factors. To investigate the spatiotemporal evolution characteristics of the trophic state and its key driving factors in Chenghai, a deep, closed lake, this study analyzes monitoring data from three regions of Chenghai Lake from 2010 to 2023. We employed M-K tests, Spearman"s rank correlation analysis, multiple linear regression models, and piecewise structural equation modeling to examine the spatiotemporal evolution of the lake’s trophic state, elucidate the direct driving effects of various factors on the trophic state, and explore their indirect pathways of influence. The aim of this study is to provide both theoretical insights and data support for the protection and management of Chenghai"s water environment. The results are as follows: (1) Between 2010 and 2023, the trophic state of Chenghai Lake showed a phased evolutionary pattern, predominantly remaining at a mesotrophic level. (2) Ecological water replenishment played a positive role in shifting the lake’s trophic state and its trends. Specifically, the comprehensive trophic state index (TLI(Σ)) in the southern and central regions showed a significant upward trend before water replenishment (2013–2018) (p < 0.01), and a significant downward trend after replenishment (2019–2023) (p < 0.05). No significant differences in TLI(Σ) were observed before and after replenishment across different regions, and the trophic state remained mesotrophic. (3) The trophic state of Chenghai Lake was significantly influenced by seasonality, with TLI(Σ) being higher during the colder spring and winter months compared to the warmer summer and autumn months. This may be attributed to the lake’s concentration effect and thermal stratification. (4) Physical and chemical factors are the main drivers of changes in TLI(Σ). Over all periods, nitrogen and phosphorus inputs, primarily from external sources, indirectly influenced TLI(Σ) by affecting other factors. Moreover, a series of ecological and environmental protection measures implemented in recent years have reduced the contribution of nitrogen and phosphorus inputs to TLI(Σ). It is important to note that despite significant ecological water replenishment, the trophic state of Chenghai Lake remains at a mesotrophic level. This indicates that the impact of terrestrial pollutant inputs and internal pollution on the lake’s trophic state should not be overlooked. Therefore, while maintaining and enhancing ecological water replenishment, a critical focus for future conservation efforts will be to reduce terrestrial pollutant inputs and remove pollutants from the lake’s water.