Abstract: The ageostrophic wet Q-vector method is applied to diagnose a torrential rainfall process in Shandong Peninsula on August 1,2001,which is caused by a temperate cyclone transformed from typhoon "Toraji"moving northward. The results show that there is a good relation between the area of torrential rainfall and the negative area of divergence of ageostrophic wet Q-vector. By advance of 6～12 hours,the divergence of ageostrophic wet Q-vector at 925 hPa makes a good index to heavy rain in Shandong Province. The ageostrophic wet Q-vector method is a good tool for forecasting the heavy rain in the province. It can give a strong signal about the torrential rainfall area in advance,and using it to forecast torrential rain area is much better than using the field of divergence and the divergence of water vapor flux. It serves as a new good way to forecast torrential rainfall in the province. The increase of temperature and humidity is more obvious at 925 hPa than at 850 hPa,which may be partly one of the reasons why the divergence of ageostrophic wet Q-vector at 925 hPa is a good index in this torrential rainfall process.