Abstract: Typhoon Haitang (2005), landed in Fujian province from 0000 UTC 19 to 0000 UTC 20 July 2005, gives rise to torrential rainfall mainly located over the northern coastlines of Fujian and Zhejiang Provinces on the right side of the typhoon track (referred to as the'R region'). Less than 10 mm of rainfall is recorded in southern Fujian and Guangdong Provinces on the left side of the typhoon track (referred to as the'L region'). There was an asymmetric structure to typhoon Haitang's rainfall during landfall. The cause of torrential rainfall and its asymmetric distribution formation is analyzed based on the data from Weather Research and Forecasting (WRF) model simulation. The results are as follows. (1) Meso-scale forcing plays the primary role in determining the ascending motion associated with torrential rainfall in such a way that large-scale forcing role is secondary. The topographic lifting always produces an ascending motion and topographic friction generates the ascending motion after the landfall. (2) The rainfall fields forced by Q vector and terrain both have asymmetric structure. Atmospheric forcing plays an important role in the formation and development of asymmetric structures, and topographic forcing plays an enhancing role. (3) Further explicit diagnostic study indicates that in the R region there is a strong upward motion, whereby plenty of water vapor is transported from the lower troposphere to the upper troposphere, which produces torrential rainfall. In the L region, the above conditions are all weaker, which may lead to weak rainfall. Finally, prospective study on the cause of precipitation from typhoon in the future is discussed preliminarily.