In this paper, a case study is performed on three small hill watersheds in Fuhe River Basin, where their storm flood process was simulated elaborately by using distributed Xin’anjiang model. The critical rainfall of each watershed is calculated by means of the verified Xin’anjiang model, so the different initial soil moisture of each watershed and the critical rainfall under different combinations of warning intervals can be determined. The calculated result of critical rainfall is demonstrated to be reasonable by comparing and validating with the measured storm flood data and storm design, which suggests that the distributed Xin’anjiang model can be used to determine the critical rainfall for early-warning of mountain flood in Fuhe River Basin. Besides, it provides a reference method for the early warning of mountain flood in Fuhe River Basin.