Abstract: Influenced by geology,geographical environment and climate,the problem of excessive pH value in Yunnan plateau lakes is prominent. In this study,combined with years of tracking monitoring data,the factors affecting the change of pH value in Dianchi Lake have been analyzed. When the concentration of chl. a is less than1500μg/L,there is a significant positive correlation between the pH value and Chl. a. When concentration of chl. a exceeds 1500μg/L,DO and pH will also decrease. The results showed that there was a significant positive correlation between the water pH value and the photosynthetic intensity of plants in the water.