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The water quality of Miyun Reservoir and the drainage area of Baihe River， where precipitation enhancement was conducted， was monitored from 2006 to 2009. The results show that the annual mean pH difference at the two monitoring points was not pronounced; the highest pH was in spring， relative higher in autumn and summer， and the lowest in winter; the downward trend of pH variation at Miyun Reservoir was more evident than that at Baihe River. The effects of reservoir storage capacity and precipitation on pH are analyzed. The results indicate that from May to September the pH decreased with the increasing of precipitation， while from October to next April there was negative correlation between pH and precipitation; the influence of precipitation on annual pH variation is more obvious than that of the reservoir storage capacity. The chemical compositions in the reservoir (except K) were lower than those in the Baihe River. Ca concentration was the highest， followed by SO2-4， and the total concentration of alkali metals (Ca， Mg， Na， K) was much higher than that of anions （SO2-4，Cl-，No-3，F-）. The concentrations of both Alkali metals and anions in winter and spring were higher than those in summer and autumn， with the maximum value in winter. The trend of reservoir anion interannual variation was not pronounced， and alkali metal variation showed a downward trend. The interannual Ag variation showed an increasing trend， and the trend in Baihe River was slightly obvious than that in Miyun Reservoir.