In this study, an optimal forest configuration for water conservation function was explored, which this optimal forest configuration could guarantee the forest vegetation can be give full play to water and soil conservation function. The remote sensing image data in 2009 to 2014 of Beijing and the data of Beijing forest resources inventory of 2009 and 2014 were used to carry out the evaluation and value estimation of water conservation function of typical forest (coniferous, broad-leaved and mixed forests) with GIS software in this paper, where in capital function core area, urban function development area, new urban development area and ecological conservation development area in Beijing. The evaluated and estimated results show: (1) the forest vegetation area in Beijing was 4.00×10⁵ hm² in 2009 and 4.59×10⁵ hm² in 2014. The average Normalized Difference Vegetation Index (NDVI) in Beijing was 0.331 in 2009 and 0.708 in 2014. The forest vegetation area in Beijing presented an overall increasing trend. The regulated water volume of forest land greening in Beijing in 2014 was nearly 1/3 lower than that in 2009, and the value of water conservation function in 2014 was 22.08% and 8.24% lower than that in 2009, respectively. (2) The annual average capacity of water conservation in the ecological conservation development area is the strongest (2.88×10⁸ m³/a), followed by the annual average capacity of water conservation in the new urban development area (1.72×10⁸ m³/a), while the annual average capacity of water conservation in the capital core area is the weakest (1.05×10⁶ m³/a). Among the coniferous, broad-leaved and mixed forests, the value of water conservation function of mixed forest is increasing, and its value of this forest type increased by 5.13% in 2014 compared with that in 2009, while the value of water conservation function of broad-leaved forest and coniferous forest decreased by 34.93% and 55.55%, respectively. (3) Cultivating pure forest to increase the forest cover rate has a very important ecological significance for regulating soil and water resources and protecting ecological environment, but considering the amount and value of forest water conservation function, this study concluded that the mixed forest had stronger water conservation function and better economic value than pure forest. Under the specific conditions of the city, the artificial plant community coordinated with ecology and landscape can make the utilization of urban land resources to reach the ecological society. The best combination of the three economic benefits is the key to improve the quality of urban green space.