Abstract:In this paper, the combined effects of forest for conservation of water source in the Qilian Mountains have been studied by fixed position observation method of forest ecological system. The results indicate that the Picea crassifolia forest is the best ecological system of water conservation forest over the Qilian Mountaions. Its mechanism for conservation of water supply are given as follows canopy can slow the raindrop splash down to the surface of soil and intercept 25.4 per Cent precipitation, the litter of forest soil is thick;the noncapillary porosity is high, forest soil structure is good, all precipitation are detained in forest soil, the surface runoff does not happen, and it can be decreased by forest vegetation. The forest vegetation can also cut down the foold peak and prolong the runoff time, which plays a role of suppling more water in dry season.