Abstract: In this study, the annual average of water quality indicators at 11 sites in Erhai Lake from 2011 to 2020 was used to identify the main factors affecting the water quality and to analyze the spatial and temporal variations of the water quality of the Lake by principal component analysis method with the help of SPSS 23.0 statistical software. According to principal component analysis, the main causes of water pollution in Erhai Lake were eutrophication and organic matter, while the water quality in the central part of Erhai Lake was the best,followed by the southern part and the northern part of Erhai Lake, and the water pollution in Erhai Lake became lighter in 2011—2014,from 2015—2018, water pollution tended to be worse, and water quality continued to improve in 2019 and 2020.