The climate fluctuation and human activities have significant effects on the changes of surface hydrological processes inthe basin, especially for the semiarid steppe basin. This paper chosen the Xilin River basin to analyze the variation characteristicsof its runoff series from 1963 to 2015 by using the improved M-K trend test method, the double accumulation curve method, thecumulative anomaly method and the wavelet transform method. The main influencing factors were analyzed by the statisticalcorrelation test and the principal component analysis at hydrological year and seasonal scale. The results show that the runoffdecreased significantly at all time scale in the Xilin River basin in the past years; the mutation year of it was in 1998, and thedominant factors affecting runoff were changed after that year; the cycles of the hydrological year and dry season runoff were 6 and25 years while there were no significant cycle changes in the wet season runoff. The factors like precipitation, evaporation, relativehumidity, and human activities have significant impacts on runoff, but the human activities are the dominant ones.