Two sensitivity simulations for the effects of the westerly and monsoon on precipitation in the Heihe River basin are conducted by using the new generation mesoscale weather model WRF(Weather Research and Forecast model)version 2.2. The horizontal current field, water vapor transportation,　 and vertical velocity are analyzed. It is found that the effects of westerly and monsoon are different. The westerly has a direct influence on precipitation in the Heihe River basin, while the monsoon has an indirect effect, through adjusting the westerly. The affecting area and intensity are also different. When westerly strengthens, precipitation in the southwest mountainous area increases and moves westward, with a maximum precipitation center moving northwestward. When monsoon strengthens, precipitation in the basin moves eastward and increases, with a maximum precipitation center shifts to southeast. Compared with other meteorology factors analyzed above, the vertical velocity shows best relationship with precipitation.