Abstract: Based on the GPS precipitation data, from Sep 22-26, 2008, the durative heavy rain fall in Chengdu is analyzed.The results show that when the PWV rises up from the bottom or drops down from the summit, corresponding with the rain starting or ending.The acuteness of risen up corresponds with the actual precipitation intensity.Using PWV 1-hour change data or 3-hour change data, it analyzes the rainfall starting, intensity and location well.But the rainfall forecasting should consider dynamical and thermodynamics conditions, it also need to combine with several exploration information to give the rainfall forecasting guideline with GPS products.