In the absence of measured hydrological data which happen flash flood disaster, the calculation of critical rainfall is particularly difficult.The paper took the little basin of Chenjiahe river tributary in Long county of Baoji as a case study,used the design local rainstorm process,analyzed runoff and flow concentration and finally got the flood process at the early warning object. Under the assumption of rainfall and runoff at the same frequency, it considered three typical soil moistures of Pa being equal to 20mm (drought) and Pa being equal to 50mm (General) and Pa being equal to 80mm (wet)critical rainfall. According to the measured channel data of Chenjiahe and using the inversion method of water level/flow,the paper got the critical rainfall and ready to transfer rainfall of watershed early warning object so as to provide technical support for the disaster warning and person transfer in Chen Jiahe small watershed.