Rapid economic and social development in the Yongding River basin has resulted in a considerable increase in water consumption, and the Beijing section of the Yongding River has been continuously cut off since 1997, causing the water ecosystem to be seriously degraded. Since 2019, an ecological water supplementation project for the Yongding River has been implemented to repair its ecosystem. Water quality changes in the Yongding River after ecological water supplementation are an important aspect of the comprehensive evaluation of the effect of the project in the Beijing section of the river. In this study, we use Sentinel2-MSI satellite remote sensing data to determine the water transparency and chlorophyll-a concentration of the Beijing section of the river in the summers of 2018, 2019, and 2020, and to analyze the spatial and temporal changes in water quality after the ecological water supplementation of the river in the spring of 2019. Compared with the values from 2018 before the ecological water supplementation, the transparency increased and the water was more limpid in 2019 and 2020 after the ecological water supplementation; the concentration of chlorophyll-a decreased, and the degree of eutrophication was reduced; variations in these water quality parameters illustrate that ecological water supplementation improved the ecotope of the Beijing section of the Yongding River.