As a case study of area in the Jinshui River basin of the upper Han River, the impacts of forest vegetation on water environment were investigated. The water conservation volumes were calculated by using the annual rainfall, crown cover interception rate and areas of different forest types. The effects of the forest ecosystem on water quality were determined by monitoring and comparing the water quality of rainfall, litter layer water, soil layer water and river water in the river outlet for the broad-leaf forest ecosystem of the basin. The results indicated that (1) the annual conserved water volumes through the forest ecosystem of the Jinshui River basin was about 466.79×106 m3. (2) The pH values could be regulated by the broad-leaf forest ecosystem of the basin, which alleviated the rain acidity. Similarly, the physico-chemical concentrations of rainfall, including TDS, CODMn, HCO3, SO4, Cl, NO3, NH4-N, NO3-N, PO4-P, TDP, As, Ba, Cr, Na, Pb, Fe, K, Mn, V and Zn, were reduced, and hence the water quality was purified. (3) The sources of matter in the river water were different based on the assessment for the water quality in different water layers. Thus, the study provided a reference for the management and construction for the water source area of the Middle Route of South-to-North Water Transfer Project, China.