This paper focuses on the upper Chao Watershed, which is the important water source for Beijing and 40% of the upper Miyun Reservoir Catchment, aiming at the analysis of the changes of rainfall - runoff relationship from 1961 to 2005 and the impacts of human activities (especially ecological construction projects, e.g. planting trees) upon the rainfall- runoff relationship. By drawing a double mass curve of rainfall and stream flow data from 1961 - 2005, 3 periods could be divided: 1961 ~1978, 1979 ~1993 and 1994 ~2005. The correlation between rainfall and stream flow kept weaken, as well as average annual stream flow kept reducing from period I to period II and III. Corresponding with the changes of rainfall- runoff relationship, the human activities in this watershed became more and more intense. In period II, most of the irrigation works and water conservancy establishments were built and applied, at the same time, the areas of forest and cultivated land increased by 7.8% and 7.5% respectively under the impacts of land use activities. In period III, forest lands increased by 11.8% as the results of afforestation and reafforestation aiming at water and soil conservancy. All kinds of human activities results in the weakened response of stream flow to rainfall and the reduced water yield. In different rainfall type years, the rainfall - runoff relationships have shown much difference. Comparing to high - rainfall years, the human activities exerted more influence on rainfall- runoff relationship in low- rainfall years.