Based on the Monitored data of the GPS network in Beijing, the relationship betw een the variation characteristics of precipitable water vapor （PWV）in the rainstorm event during 2004 flood season and its rainfall and intensity was analyzed . The results show that the increase of PWV was often restricted by the local weather system, when weather s ystem in ground and upper air picture make for the transportation of warm and moisture a ir, isohume denseness section was formed nearby Beijing, the PWV was increas ing during 1314 hours before rainstorm appearance; when there was convergence system moving eastward or mesoscale weather system forming, PWV increased s ignificantly during 34 hours before rainstorm, heavy rain appeared after value of PWV above threshold, but the value of PWV and increment did not corr espond to the rainfall.