The circulation patterns of the heavy rain process in Huabei area on August 1996 was analyzed. The results show that the gradient between the subtropic high and typhoon is so strong that broad southerly jet continuously transports the water vapor and the energy to the north. The Mount Taihang was just located in convergence area, so it formed the advantageous weather situation for rainstorm. Based on MM5 numerical model, in term of the synoptic principle and the potential vorticity theory, the rain process of 4-5,August 1996 was simulated and its stability was analyzed. The main results are as follows: (1) MM5 model has a good performance to simulate the typhoon rainstorm physical process. (2)In this rain process, the instability stratification in South is higher than that in North, and there are symmetry instability and convection instability simultaneously. In addition, the conditional symmetric instability not only causes the circulation acceleration, but also increases the precipitation.