【Objective】Climate change coupled with anthological activities could alter the hydrological cycle of catchment.The purpose of this paper is to analyze the factors affecting the runoff in Lan river catchment within Shanxi province.【Method】The analysis was based on hydrological simulation using the RCCC-WBM model.【Result】The RCCC-WBM model worked well under natural condition with the Nash-Sutcliffe coefficient being more than 80% and the relative errors less than 2%.The measured runoff from 1969 to 2015 from the gauge at Shangjingyou station showed a decrease by more than 50% since 1968,76.1% of which was ascribed to human activities.Due to the rise in precipitation in this area over the past few years,the decrease in runoff induced by climate changes became less significant.【Conclusion】Human activities are the main reason behind the decrease in runoff in the Lan river basin.