Human activity and climate are two main factors directly influencing catchment runoff, and separation of their impacts is of great importance for land use planning and water resource management. This study was designed to examine the impacts of climate change and human activity on stream flow characteristics. Using the nonparametric Mann-Kendall test and moving t-test, we accessed the trends and change points of the runoff, precipitation, and potential evapotranspiration data series in the watershed. And Zhang’s function was employed to evaluate the impacts of variation in climate and human activity on mean annual stream flow. The results showed a significant downtrend and change points in annual runoff, with an abrupt change identified in 2001 in the whole watershed, and the stream flow data was divided into a baseline period and a period of change. The decrease in mean annual runoff between the baseline and the period of change indicated that climate change accounted for about 33.1% of the total decrease, and that human activity was responsible for about 66.9%. This study concluded that human activity will significantly impact water resources in the Loess Plateau region, and scientific management measures should be implemented in the future.