Abstract: Four typical forest types in in Beijing Songshan National Forest Park were selected as the research objects in this study. By collecting the dimensional administrative levels under different forest types(canopy layer, litter layer and soil layer) water and river water sample, groundwater sample and reservoir water sample, and take them back to the laboratory to test and analyze the ion concentration in each water sample. The variation trend of ion concentration in rainfall input-output process was discussed, calculated the influence degree of the 4 stands in Songshan National Forest Park on the water quality. The results show that from forest canopy layer to the soil layer, the change of Na⁺, Mg²⁺ and Ca²⁺ concentrations showing a rising trend, while the changes of K⁺, NH₄⁺ and Cl⁻ concentrations presents a trend of increasing first and then decreasing, the turning is 0–20 cm soil layer respectively, the forest canopy and litter layer, while the concentrations of NO₃⁻, NO₂⁻, SO₄²⁻ and F⁻ change presents down first, then the trend of rising and falling, the turning is litter layer respectively and 0–20 cm of soil. The 4 typical of forests have an obvious effect on purifying the stream water, the streams of Na⁺, K⁺, NH₄⁺, NO₃⁻, NO₂⁻ and SO₄²⁻ concentration comparison with the soil have been decreased by 13.54%–86.81%, and less influence on groundwater and water in reservoir. On the basis of Environmental Quality Standards for Surface Water and Quality Standard for Ground Water, comprehensive evaluation results of water quality showed that 20–60 cm soil layer played a key role in purifying the rainfall water quality, and the purification effect of Quercus mongolica pure forest was the best. Among the 4 typical of forests, Q. mongolica pure forest has the best effect on water quality purification effect, followed by Pinus tabuliformis + Populus tomentosa mixed forest, Pinus tabuliformis pure forest and Populus davidiana pure forest.