Improved analysis and usage of water resources in the Taoer River basin requires an evaluation of the contributions of precipitation and human activities to runoff. In this study, we apply an integrated method combining the non-parametric Mann–Kendall trend test and the double-mass curve to analyze runoff data from 1961 to 2010. The major findings are as follows: (1) annual runoff showed a statistically significant decrease, while precipitation showed no significant trend; (2) an abrupt change point was identified in 1998 at four representative stations, resulting in the study period being divided into pre-change and post-change periods for subsequent analysis. The double-mass curves were approximately linear in the pre-change periods, indicating that the dominant factor was probably climate change. Annual precipitation–runoff curves showed a decreasing trend from 1998, probably because of human activity; (3) the contributions of human activity to runoff in the post-change period for the four selected stations were 58.31%, 17.81%, 37.17%, and 47.66%, and the influence of human activity increased after the abrupt change point.