Abstract: Taking Beizhuang village of Miyun county for a experimental site, based on investigation to water conservation forest and observation of surface water, the main problems and causes of water quality were analyzed. Since engineering of reuse farmland for afforestation of Miyun reservoir was put in practice in 2000, forest area had increased, and non point sources pollution caused by field fertilization had been reduced, which led muddy degree of surface water and the content of NH + 4 and Tur to decline. In the case of Pinus tabulaeformis Carr forest and Castanea mollissima Bl forest, water quality of runoff produced by precipitation which flowed in forestry eco system was supervised and analyzed. The results showed the process of leaching, exchange, adsorption, etc. between water conservation forest in valley of Miyun reservoir and precipitation can cleanse precipitation.