In order to understand the regularity of air PWV (Precipitable Water Vapor ) variation after rocket cloud seeding, by means of the observation data obtained by the singlestation groundbased GPS at Qinhuangdao, Hebei Province, two cases of rocket precipitation enhancement operations in upwind of the GPS station are analyzed. It is found that the air PWV over the GPS station decreased while the rainfall intensity increased 1 to 3 hours after each rocket seeding operation. The PWV rose again and was compensated faster when seeding at the early stage of weather system development than seeding at the late stage, and seeding effectiveness is more significant. The observation results are compared with some early and new numerical simulation conclusions.