Quantitative evaluation the effects of climate change and human activities on river flow is of great importance for water resource management and utilization. As the water source of Chinas South-to-North Water Transfer Project in the middle line, Danjiangkou reservoirs function may be severely affected by the decrease of runoff into the reservoir. In this paper, runoff data from 1960 to 2012 and climatic data from 20 meteorological stations of Hanjiang River basin between 1960 and 2012 were used to analyze and compare the changing trends of runoff, precipitation and temperature by using the Mann-Kendall test method and the Pettitt test method, aiming at finding reasons for the decrease of reservoir inflow. Then, the climate elasticity model is adopted to estimate the degree of climate change effect on flow decrease. Our results indicated that climate change was the dominant reason for the inflow decrease in spring and autumn. In spring, climate change accounted for 67% of the decrease of reservoir inflow and human activities accounted for 33%; in autumn, climate change accounted for 88% of inflow decrease and human activities accounted for 12%.