Climate change and intensive human activities had profound impacts on the global water circular system. As an important part in the water circular system,runoff changed significantly due to these two factors. Taking the Xitiaoxi Basin,which is located in the upper reaches of the Taihu Basin,as study area,the hydrological and meteorological trends in the Xitiaoxi Basin from 1972 to 2015 were analyzed by employing the accumulative anomaly,linear trend analysis and the flow duration curve method. Without considering the effect of evapotranspiration,the slope change ratio of cumulative quantity ( SCRCQ)method and the climatic elasticity model were adopted to estimate the impacts of precipitation and human activities on the runoff change quantitatively. The main results were showed as follows: ( 1)A decreasing trend of annual runoff was revealed during 1972-2015 in the Xitiaoxi Basin. An abrupt change was identified around the year 1999,then the entire period was divided into two parts: baseline period ( 1972-1998)and variable period ( 1999-2015). The annual runoff in the Xitiaoxi Basin decreased by 11. 76% or so after the occurrence of the abrupt change,which changed greatly. ( 2)The annual variation tendency of precipitation was not significant. Compared with the period before 1999,the annual average precipitation decreased by 1. 43% during 1999-2015,which changed lightly. The runoff in the Xitiaoxi Basin was very sensitive to the change of the precipitation. The proportion of the flood season precipitation in the annual precipitation has been declining and the seasonal distribution of precipitation gradually became smooth,which contributed to the runoff decrease in the Xitiaoxi Basin to a certain extent. ( 3)According to the slope change ratio of cumulative quantity method,the contribution ratios of the precipitation change and human activities were 26. 45% and 73. 55% respectively,while the results obtained by the climatic elasticity model were 23. 52% and 76. 48%. The results of these two methods were close and indicated that human activity was the main factor that led to the runoff decrease in the Xitiaoxi Basin during 1972-2015.