The modified Q-vector methed was applied to diagnose the torrential rain in Shandong during 11—13 August,1999.The results show that the Q^*-Vector towards the torrential rain are and the position of the Q^*-vector convergence at 850 and 700hPa is correlated well to the ascending motion and torrential rain area.It is found that storm rainfall occured in the warm air side of the Q^*-Vector moist frontogenesis zone at 850 and 500hPa.