Abstract: In order to enhance the ecological health of the Yongding River in Mentougou District, to guarantee the ecological water demand of the river, and to address the impacts of climate change and human activities on the river ecosystem, the current study analyzed the characteristics of the evolution of hydro-meteorological elements and the time series dynamics of the ecological water demand of the river. In accordance with the ecological protection objectives of the Yongding River in Mentougou District, five river ecological water demand components, namely ecological base flow, aquatic life water demand, sediment transport water demand, self-purification water demand and water surface evaporation, were integrated and calculated using the environmental function setting method. Furthermore, the factors affecting the river ecological water demand were determined by quantitatively analyzing the impacts of climate change and human activities on runoff. The results indicated that the average annual temperature in the study area exhibited a significant upward trend from 1980 to 2020, while the annual precipitation exhibited a slight increase but not to a statistically significant degree. The ecological water demand of the river in the region demonstrated a downward trend from 1980 to 2020, with the aquatic water demand representing the largest component. It can be concluded that human activities represent the most significant factor influencing the ecological water demand in the region, which in turn affects the runoff volume and subsequently the ecological water demand of the river. The findings of this study may serve as a theoretical foundation for future water allocation, water resource management, and the prediction of the ecological water demand of the Yongding River in Mentougou District.