Abstract: The eutrophication is a serious problem in Dianchi Lake, algae blooms occur occasionally. The pollution control of Dianchi Lake has been attached great importance by governments, especially in the recent ten years, and the water pollution control in Dianchi Lake has been vigorously promoted. Based on the monthly water quality data of state-controlled water quality monitoring section in Dianchi Lake in recent 10 years (from 2011 to 2020), this paper studied the spatio-temporal variation characteristics of water quality in Dianchi Lake. Dianchi Lake is divided into Caohai Lake and Waihai Lake by Haigeng dike. The concentrations of chlorophyll a (Chl-a), total phosphorus (TP) and total nitrogen (TN) in Caohai Lake area were 0.002~1.012 mg/L, 0.030~0.718 mg/L and 0.20~9.97 mg/L respectively. The concentrations of chlorophyll a (Chl-a), total phosphorus (TP) and total nitrogen (TN) in Waihai Lake area were 0.003~0.589 mg/L, 0.015~0.491 mg/L and 0.35~9.70 mg/L respectively. The concentration of Chl-a in Dianchi Lake was significantly correlated with BOD5 and TP. Based on the hierarchical clustering analysis of water quality characteristics, the 10 monitoring points in Dianchi Lake were divided into two groups (Caohai Lake and Waihai Lake). The water quality of Caohai Lake was significantly worse than that of the Waihai Lake. The TP in the Caohai Lake water area showed a continuous decreasing trend, and the TN showed a fluctuating decrease from 2011 to 2017, while the fluctuation of the total nitrogen concentration increased from 2018 to 2020, and the trophic level index (TLI) decreased significantly from 2016 to 2020 compared with that from 2011 to 2015. The TP in the Waihai Lake showed a continuous decreasing trend, while the TN showed a fluctuant decline trend. The TLI showed a decreasing trend from 2011 to 2016, and showed a decreasing trend first and then rising trend from 2017 to 2020. In general, the water quality and trophic level of Dianchi Lake had improved gradually in the past decade, but the TN had an increasing trend in recent years, and the concentration of Chl-a was still high in the whole lake. It is suggested that measures should be taken to coordinate the control of TN on the basis of the control of TP, and promote the restoration of the structure and function of Dianchi Lake aquatic ecosystem.