Abstract: Typhoon Haitang landing Fujian Province from 0800 LST 19 July to 0800 LST 20 July in 2005, gives rise to a significant rain process during the 24 hours, and precipitation intensity on the north side of typhoon is stronger than that on the south side, which shows obvious asymmetric distribution. The rainfall process is simulated using the WRF model and based on simulated outputs, the relative humidity and vertical ascending motion fields are analyzed, meanwhile, the forcing roles of orgoraphic lifting and surface friction are calculated and the diagnosis of the revised wet Q vector is conducted, so the cause of asymmetric distribution formation of rainfall associated with typhoon Haitang is analyzed quantitatively. The results are as follows: (1) The rain asymmetric distribution feature, the fallout region of primary precipitation, and the intensity and the location of extreme rain exceeding 300 mm are simulated successfully, so the simulation effects are considerably inspiring. (2) Vertical ascending motion condition may be the predominant factor leading to asymmetric distribution of precipitation associated with the typhoon. (3) The convergence intensity of the revised wet Q vector divergence on the north side of the typhoon is stronger than that on the south, and from the further calculation and analysis it is found that the rain field forced by the revised wet Q vector divergence also shows marked asymmetric distribution, with intensity on the north stronger as compared with that on the south side of the typhoon. (4) The precipitation intensity forced by orographic factors is about 1.6－2.5 times of the counterpart caused by the revised wet Q vector divergence, and the rain intensity forced by surface friction is approximately 2－3 times of the counterpart caused by orographic lifting.