Analysis of the main driven factors which caused runoff variations can provide significant information for water resources and ecology researches. Based on the runoff and meteorological data with series from 1970 to 2014 of a typical steppe watershed Xilin River,Mann-Kendall and Elastic Coefficient Method were used to quantify the contribution of climate change and human activities on runoff changes. The results indicated that the annual runoff present a significant decrease trend during recent 45 years,and occurred mutations in the years of 1979,1986 and 1998,respectively. For the period of 1980 to 1986,the contribution rate of human activities accounted for 46.57%,which reached 91.46% for the period of 1987 to 1998 and climate change just had a percent of 8.54%. For the period of 1999 to 2014,66.16% of the runoff variation was caused by human activities. In brief,human activity is the main driven factors which induced runoff changes.