Abstract: In this paper, the effect of protecting water source of forest is analyzed by using "large basin comparision method." The research area belongs to semi-arid stony mountain areas which located in the midtemperate zone. The two large basins are West River Basin and East River Basin which are fourth stream order in headwaters of Yong-Ding River. The influence and controlling function of forest on streamflow are analyzed on the basin of comparability investigation. In accordance with above analysis, it can be concluded that:1. Under the influence of forests, the runoff of low water increased by 80% and flood peaks decreased by 80%, thus uniformity of streamflow within a year increased by 40%. Furthermore forest can increase total annual rainfall.2. Because forest can improve soil and increase non-capillary porosity in soil, controlling capacity increased by 24.1% and infiltration capacity increased by 48% for the rain in soil layer (within 1 meter) in the basin.