In the evening of July 27th, 1984, a whole process of a cumulonimbers precipitation was observed in Peking University, which had two developing processes and two declining processes. The first process of precipitation lasted a short time(33min) and had a high average rainfall density (12.9mm/hr). The pH value of the first precipitation was about 7.0.The second process of precipitation kept a long time(126min) had a low average rainfall density 1.6mm/hr, With the strong washout of precipitation, the atmosphere under the cloud became cleaner. The pH value of the second precipitation dropped 6.6 continually and the conductance became down markebly.