In regard to the strong convection clouds from afternoon to night on 8 August 2008 in the northwest, west and southwest of Beijing area, weather modification activities were conducted to find out the changes of the surface humidity leeward after the largescale operations. In this paper, by means of the data obtained from 215 automatic as well as nonautomatic weather stations， the precipitation distribution was studied. Especially, by means of the data obtained from 108 weather stations with the humidity detection, the ground water vapor pressure every 5 min is calculated and analyzed, taking aim at the experimentation evidence for the largescale modification operations. The study shows that the leading airflow in the evening was southwest to northeast, and the weather system moved basically along the direction of southwest to northeast. Largescale weather modification operation was conducted in the southwest part of Beijing area. After the operation, in the zonal belt of the operation, the ground water vapor pressure changed with time abruptly (it suddently diminished after lasted about 25 min). The break change weakened from southeast to northwest and the start time of the change postponed. In the axis line of the zonal belt, the break change was smallest. Farther away from the axis line, the smaller the break was. Outside the zonal belt, such phenomenon did not occur. It was as if a dry air column moving from southwest to northeast. Without the weather modification activities on 8 August 2008, there would be no such phenomenon in the rain band backward position. Meanwhile, the causes for the above phenomena are analyzed, and some useful operational instructions are obtained.