Studies on the attribution of runoff variation under climate change and human activities are valuable to understanding runoff change. This paper analyzes the quantitative contribution of climate change and human activities to runoff variation, using the Budyko assumption and TOPMODEL with application to a case study of the Beiluo River basin of the Wei River. We have examined 25 scenarios of temperature and precipitation changes in the possible range of climate changes, and analyzed the influence of all these scenarios on the runoff. Simulation results show that historic precipitation and runoff took a decreasing tendency while temperature had a rising trend and both temperature rising and precipitation decreasing made contribution to runoff change. But human activities were the major cause for runoff decreasing over the last 50 years and this had a contribution rate up to 58.9% and 65.2% by the two methods respectively. Under different scenarios, the variation in monthly runoff shows a wide range and the influence of precipitation change is stronger than that of temperature change, indicating precipitation change as a major factor of the future variation in water resources.