Abstract: Plateau-closed lake ecosystems are highly sensitive to environmental disturbances due to their limited water exchange and weak buffering capacity. As a core parameter characterizing aquatic chemical equilibrium, the dynamic variation of pH directly reflects the health status of lake ecosystems. This study focuses on Qilu Lake, a typical eutrophic plateau lake on the YunnanGuizhou Plateau. Based on water quality monitoring data from central, northern, and southern sites of Qilu Lake between 2015 and 2024, combined with statistical analysis and mechanistic inference, this research reveals the spatiotemporal dynamics of pH and its driving role in the eutrophication process. The results indicate significant diurnal and seasonal variations in pH, with summer afternoons exceeding pH 9.5 and winter averaging approximately 7.6. pH showed significant positive correlations with water temperature, dissolved oxygen, and chlorophyll a（r > 0.57）. Algal photosynthesis serves as the direct driver of pH elevation, while water temperature exerts a synergistic effect by enhancing algal metabolism and reducing CO₂ solubility. The study further identifies a positive feedback loop of “algal proliferation – pH elevation – endogenous phosphorus release – intensified algal blooms.” When pH > 9.0, sediment phosphorus release rates increase significantly, while high pH promotes the proportion of non-ionic ammonia（NH₃）, exacerbating ecological toxicity. Thus, pH transitions from an “indicator” of lake health to a “catalyst” driving eutrophication. Accordingly, this study proposes establishing an algal bloom early warning system with pH as a core indicator and implementing differentiated management strategies based on ecological regime recognition.