In order to understand variations of pH and chemical composition of precipitation in Beijing, 5 precipitation events in summer time of 2007 were sampled step by step sequentially on time, and the variations of pH, EC and the characterization of water-soluble ion such as NH4+, SO4(2-) and NO3(-) were analyzed. The results showed that pH was 5.70 +/- 0.73 in the beginning (before-35 min) and the precipitation was not acidic, but in the steady period (after-35 min) pH was 4.35 +/- 0.56 and the raining water presented acidity actually. Simultaneously, pH, EC and concentration of ions decreased rapidly with the raining continuously, 10-45 min later that became nearly constant and the concentration of water soluble ions decreased 50%-90% compare to beginning. The major pollutants in the precipitation on Aug. 1, which the raining air mass came from the northwest, was low and the concentrations of NH4+, SO4(2-) and NO3- were 65.4, 23.9 and 117.3 microeq/L, respectively; while the major pollutants in the precipitation on Aug. 6, which the air mass came from south, was high and the concentrations of NH4+, SO4(2-) and NO3- were 310.8, 95.7 and 249.8 microeq/L. But acidic precipitation was much higher when the air mass came from south than from northwest. The increasing fine particles from photo-chemical reaction in summer time of Beijing will result that the precipitation of the rain will be more and more acidic.