The forest in Qilian Mountain area has a multiple ecological function of soil and water conservation，regulating the rainfall and snow melt water in the hilly region of Qilian Mountain and the surrounding area，irrigating the Hexi Corridor，and maintaining biodiversity，hence plays a significant role of being an ecological barrier. The existing forest area in Qilian Mountain area is 436，000 hectares and the forest coverage rate 16.4% among which Qinghai spruce tree occupies the largest percentage in terms of distribution. Tree-ring-data-based study shows that the Qinghai spruce tree in the Qilian Mountain region grows faster than before after 2000 along with the regional climate warming，rainfall increasing and the extension of the growing season in recent decades，at the same time the change of the normalized difference vegetation index （NDVI） also shows a good trend of the regional vegetation as a whole. The tree-ring data of Sabina chinensis in the Qilian Mountain region reveals that the climate in the Qilian Mountain region has gradually become humid in the past nearly two hundred years thus providing a relatively suitable environment for tree's growing while taking the past one thousand years into our consideration. The research on Alpine-forest-line which is the most sensitive part of the Qilian Mountain forest ecological system tells that the climate warming caused the alpine forest line has a tendency of climbing-up，the density of trees at Alpine-forest-line increases quickly. The comprehensive analysis on the existing tree-ring data of threes in the Qilian Mountain region displays that the tree growth as a whole in this region is mainly affected by rainfall or drought factor，also significantly affected by regional climate change in short term（the diurnal，monthly and annual） and long term （decadal）. The evaluation of the ecological function of the forest in the Qilian Mountain region shows that the soil and water conservation is the most important ecological function and value of the forest in this region. The main mechanism of soil and water conservation is forest canopy interception，hydrological regulation of surface litter-layer，forest soil infiltration and regulate the surface water and river flow. We find out through comparative study that Qinghai spruce forest is the best forest type in the Qilian Mountain region in the aspect of water conservation benefits，should focus on the protection and development of this type of forest in the future.