As one of the important water supply areas of Xi''an city, the Bahe river plays an important role in maintaining urban development and ecological health．In the past few decades,the decreasing runoff of the Baihe river basin associated with climate variation and human activities had caused a series of water and eco-environmental problems. The paper quantify the impacts of climate variation and human activities on stream runoff in the source area of the Bahe river,based on analysis of hydrologic changes of the upper Bahe river basin from 1966to 1975and from 1991to 2000．It use tessellation polygon analysis, inverse distance weighting (IDW) interpolation method and rainfall-runoff modeling method to distinguish the effects of climate variation and human activities. The quantification results indicate that climate variations account for 45.4％of decrease to the stream flow, while human activities account for 54.6%. Therefore, human activities and climate variations play an equal impacts on the decline of the stream runoff in the source area of the Bahe river．Climate variations play a domain impact on the temporal and spatial distribution of precipitation, which may cause more concentrated and stronger raining.