In this paper, the area rainfall of the lower reaches of Puli RIVER in the Three Gorges Reservoir area is calculated by using the Thiessen polygon and arithmetic average methods respectively. The results are input into the FloodArea model to simulate the same torrential flood process. The simulation results are compared and analyzed with the actual flooding time and depth. It turns out that in terms of the simulation of the torrential flood process, the depth of storm water simulated by the area rainfall data calculated by the Thiessen polygon method is closer to the actual data than that simulated by the area rainfall data obtained by the arithmetic mean method. Also, the former is more consistent with the actual situation than the latter. Further, the critical area rainfall calculated by the Thiessen polygon method is more valuable for application. This method can be used to calculate the critical area rainfall caused by mountain torrents.