Based on the data of Category Ⅱ of the sixth forest inventory in Beijing(2004), the water conservation of the forest ecosystem was estimated by means of the regional water balance method as well as on the soil-water storage ability. The characteristics of water conservation of different forest types in different locations were analyzed. The result showed that a volume of 1.26×109m3 water was conserved in the forest ecosystem. Huairou District, Yanqing County, Miyun County, Mentougou District and Fangshan District were the main contributors of water conservation, and their cumulative ratio reached 63.3%. Broadleaf forest and shrub forest contributed 80.8% of the water conservation in Beijing forest ecosystem. Water conservation of forests located in the region differs with attitude. The forests with average elevation of 0-100m，100-500m and 500-800m contributed 29%, 30.7% and 23.3%, respectively. Different slope location also has an effect on water conservation, and those forests located in the flat ground and in the hills contribute with 37.12% and 40.7%, respectively.