The AgI seeding experiments using seeding aircraft were carried out to enhance precipitation at the upriver catchments of Miyun Reservoir from May to September. The regular and continuous measurements were conducted at two sampling spots located at the entrance of the Baihe River and the the reservoir, and by using of the monitoring data, an analysis is made of the temporal and spatial characteristics of water quality in the reservoir from June 2004 to December 2006. The measurements show the same chemical composition and variation trend at two sampling spots. The Ag variation characteristics of water in the reservoir were studied, and the results show that the monthly Ag concentrations changed more greatly during the seeding period; the average Ag concentrations during the seeding period were lower than those during nonseeding period; and there was a decrease tendency year after year. The chemical features are not obviously different at two sampling spots. The negative ion concentration of SO42－ in the reservoir is the highest, then Cl－；Ca and Mg have the highest concentrations in alkali metals. The results suggest that precipitation enhancement does not affect the water quality of Miyun Reservoir.