Continuous supply of water vapor is a necessary condition for the occurrence and development of a rainstorm. The groundbased GPS technology provides a new method for the detection of tropospheric water vapor, which provides technical guarantee for the accuracy and precision of weather forecast. The Bernese 5.0 software was used to process the groundbased GPS data in a heavy rainstorm on 21 July 2012 in Beijing.The temporal and spatial variation series of the PWV (Precipitable Water Vapor) are obtained and contrasted with radiosonde data. The results show that high spatial and temporal resolution groundbased GPS technology has obvious advantageous for the detection of atmospheric water vapor in the troposphere. The high correlation with the actual rainfall shows a positive significance of groundbased GPS technology in the monitoring and earlywarning of rainstorms. The results indicate that when higher PWV （>40 mm）continues in Beijing, within 24 to 36 hours there may occur a heavy rainfall; when PWV becomes very rich (increased 10 to 20 mm per hour) with sustainable PWV development in short time, a storm may be coming. The effective application of GPSMET technology in weather forecast can improve the precision and accuracy greatly.