This paper is based on the long-term monitoring data of Lake Erhai from 1971 to 2020. Temporal trends of TN, TP, NH₃-N, SD, COD_Mn and Chl.a were quantified using M-K test methods and GAMs models. Combined with multiple linear regression, relationships between water quality parameters and driving factors were analysed. The results showed that in the past 50 years, under the dual drive of watershed development and lake protection, the water quality of Lake Erhai fluctuated significantly from year to year, and the concentration of key water quality indicators also showed a series of significant change trends. The trend of the concentration of key indicators increased significantly from the early 1980s to 1985, while a decrease occurred in 1986-1989, corresponding to the development of urbanisation and the construction of main sewers in the south of the basin. Between the late 1990s and 2003, the concentration of the main indicator increased significantly again, related to factors such as the development of southern cities and industries, and the adjustment of the structure of agricultural planting. Then, the local government took some constructive measures to combat the deterioration of water quality, such as sewage treatment and ecological restoration of the lakeshore. During this period, Lake Erhai experienced two severe algal blooms. In 1996, a Herba houttuyniae bloom occurred, although the overall water quality was in the second phase, which was closely related to long-term low water operations, cage fish farming and the introduction of whitebait fish. In 2003, the second algal bloom occurred due to increased water pollution and an unrestored ecosystem. Two algal blooms threatened the sustainable ecosystem functions of Lake Erhai, which motivated the implementation of subsequent social policies and lake protection strategies. In particular, high-intensity lake protection since 2016 has contributed to the positive trend of Lake Erha's key indicators. However, it is worth noting that the COD_Mn concentration still increased significantly in the last decade, and the contribution of TLI (COD_Mn) to TLI (Σ) increased from 6% in 1992 to 17% in 2020. This indicates that the influence of COD_Mn on the eutrophication of Lake Erhai is gradually increasing, which will be the focus of future protection of Lake Erhai.