[Objective] The main contributing factors to changes in lake and reservoir water quality in plateau mountainous areas were accurately analyzed in order to provide a scientific reference for the prevention and management of environmental water pollution in these areas as well as for the entire country. [Methods] Land use data for the “Two Lakes and One Reservoir” watershed in Guiyang City in 2013, 2016, and 2019 were interpreted, and the water quality data of total nitrogen (TN) and total phosphorus (TP) were collected during flood and dry seasons. The correlation between three periods of land use/landscape index and water quality data were revealed through Pearson correlation analysis, and the contribution of three periods of land use/landscape index to water quality were identified by RDA analysis. [Results] ① Forest land, construction land, and grassland increased by 11.72%, 9.96%, and 3.18%, respectively, while paddy land and dry land decreased by 8.77% and 12.07%, respectively, from 2013 to 2019. Both forest land and construction land showed an increase in the degree of integration and in the proportion of number of patches. ② The water quality of Hongfeng Lake reached the national water quality standard classes of Ⅰ to Ⅲ. The concentrations of TN and TP in Baihua Lake and Aha Reservoir fluctuated greatly, and the water quality was worse than that of Hongfeng Lake. ③ The proportion of forest land area and landscape index were negatively correlated with the water quality index, and the proportion of construction land area and landscape index were positively correlated with the water quality index. ④ The contribution of forest patches reached 54.0% and 52.5% in 2013 and 2016, respectively, and the contribution of the proportion of construction land area reached 68.7% in 2019. [Conclusion] Forest land produced a “sink” effect, while construction land produced a “source” effect. Increasing the “sink” effect and restraining the output of pollution “sources” are effective means to alleviate the deterioration of water quality in the “Two Lakes and One Reservoir” watershed.