Abstract: Based on the measured data of 10 water quality sampling points in Dianchi Lake, this paper used the measured water quality indicators of the long time series from 1990 to 2020. Through the inter annual changes of the indicators, and comprehensive time-space analysis and statistical methods, the time-space evolution of water quality in Dianchi Lake was compared from the inter annual scale. The 2 spatial regions of Caohai and Waihai were analyzed by spatial interpolation method. The results show that the Waihai water quality index is relatively stable, the Caohai related index changes greatly, the Dianchi Lake is greatly affected by chemical oxygen demand(COD), and the correlation of Waihai water quality index is weak. From 1997 to 2015, the environmental degradation and restoration process of Dianchi Lake was carried out; after 2016, the water quality problems improved. During the period from 2000 to 2010, the content of relevant indicators(TP, TN, NH₃−N, etc.) was easily changed in the southern part of Waihai in Dianchi Lake, and the regional content was more concentrated, and the content of water quality indicators in the area near Caohai was significantly higher. The eutrophication of Caohai water is serious; although the eutrophication of Waihai is relatively moderate, the eutrophication index is at the edge critical. The CWQI values of the entire Dianchi Lake waters ranged from 64.72 to 92.38, fluctuating within the range of pass, moderate and good water quality.