The quantity of ecological use of water was determined using experimental methods in the four dominant mountain forests in Beijing area during the growing season of the year 2010. The ecological use of water Platycladus, Pinus tabulaeformis, Robinia pseudoacacia, and Quercus variabilis were 376.15, 335.28, 258.90 and 374.94 mm, respectively. Vegetation transpiration was the major water use in total eco-logical use of water. Among the four forests, the effective precipitation in Robinia pseudoacacia was able to offset the water use as evapotranspiration, and was even able to add water into the soil as water storage, while the effective precipitations in the other three forests met only approximately 75 % of the water needs as evapotranspiration and 55 % of the total ecological use of water. Therefore, Robinia pseudoacacia forest was recommended for afforesting efforts in the areas with low precipitation.