Abstract: In the Yinshan Mountain area of Bayannur, mountains are staggered and gullies are crisscross, among which there are 104 mountain flood gullies with serious flood hazards. These gullies are featured with large longitudinal slopes, short flow paths and serious hazards, and hence the flood prevention and early warning capacity needs to be improved promptly. This paper proposes a method for calculating the areal rainfall of mountain flood gullies so as to predict and warn the occurrence of mountain flood. The calculation depends on two key technologies. The first is the extraction of the boundary of the mountain torrent basin, and the second is the calculation of grid rainfall. For the former, we use the multi-flow DINF algorithm to generate surface vector data for flash flood basins. As for the latter, we use the interpolation method of distributing weight according to distance to interpolate the rainfall of the automatic weather stations to the 1.0 km grid point. The watershed areal rainfall can be calculated by intersection of the watershed areal vector and the grid rainfall. And hence the early warning of mountain flood can be determined by the amount of the areal rainfall. Meanwhile, it is found that the formation process of flash flood is affected and restricted by many factors, such as instantaneous rainfall intensity, mountain vegetation and instantaneous infiltration intensity. It is a very complex nonlinear process. Therefore, when the basin areal rainfall is used for the release of mountain flood warning, it is also necessary to make a comprehensive decision in combination with the hydrology, geography, hidden danger points and other conditions in the basin. This paper simulates the basin areal rainfall for the flood in Xiwugaigou and Dongshengmiaogou based on one case of mountain torrent with manual observation, and verifies the applicability of the basin areal rainfall calculation method proposed in this article.