Using the GRAPES_Meso model, the numerical simulation, topographic sensitivity experiments and vapor sensitivity experiments are carried out for the extremely severe rainfall that happened in Beijing Region from 21 to 22 July 2012. The conclusions are as follows. In the process of the severe rainstorm, the convergence of upward current and vapor is reinforced by the terrain effect of Taihang Mountain, thus reinforcing the intensity of precipitation. Due to the convergence of southeast, southwest and north winds in the lower troposphere, the intensity of cyclonic vorticity is strengthened, which is helpful for the vortex to stay above Beijing for about 5 hours. The topographic relief and altitude of mountain have great influence on rainfalls, and the effect of topographic relief is greater than the altitude. In the process of the rainstorm, small changes of the vapor condition lead to significant difference in vapor transportation. The vapor condition influences the rainfalls markedly.