Abstract: By analyzing the concentration changes of DO and BOD₅ in Erhai Lake from 2015 to 2019, the relationship between DO, BOD₅ and temperature and rainfall was studied. The results indicated that the content of organic pollutants that can be decomposed by aerobic microorganisms was low in Erhai Lake. The BOD₅ value of Erhai Lake from 2015 to 2019 was between 1.0~3.2mg/L, and BOD₅ showed strong stability. It was not affected by temperature and rainfall. From 2015 to 2019, the monthly average concentration of DO in Erhai Lake was 5.47~8.47mg/L, the quarterly average was 5.98~8.47mg/L, and the annual average was 6.86~7.44mg/L. When the temperature fluctuated obviously, the relationship between DO and temperature was obviously negative, but the impact of rainfall on DO was not significant. The five engineering systems constructed by Dali Prefecture Party committee and state government in the "eight key battles" of Erhai Lake Protection and governance are important factors for Erhai Lake to maintain BOD₅ at class I water quality and DO at class II water quality.