Based on the watersheds controlled by Qixiaying and Jungar Hydrological Stations of the typical tributaries within the Inner Mongolia Section of the Yellow River Basin,the contributions from the runoff variation,climate change and human activities to the runoff variations therein are quantitatively analyzed with the comparison method of the change rate of accumulation slope. The result shows that the runoff has a slowly decreasing trend for many years,which is increased with the increase of precipitation and decreased with the rise of average temperature. The runoff is minimum in 2000s, which is reduced by 45. 67% ?58. 26%,if compared with that in 1990s. The point of abrupt change of the runoff occurs in 1981. Comparing that in 1981 ~ 2010 with that in 1957 ~ 1980, the contribution rates of the climate changes in the watersheds controlled by both Qixiaying and Jungar Hydrological Stations to the reduction of the runoff are 12. 80% and 23. 46% respectively, while the contribution rates of the human activities to the reduction of the runoff are 87. 20% and 76. 54% as well.