Early-warning index is important in mountain flood warning. This paper put forward a method of determining rainfall warning index with the Datuan River in the Laoshan Basin as a case. According to the different degree of soil moisture, this paper also obtained the relationship between rainfall and runoff based on the rainfall -runoff curve， used reasoning formula method to calculate the design flood， and drew the rainfall flood peak curve. According to the investigation in the Datuan field of Laoshan， Manning formula was used to get water level -flow curve， and calculated critical flow. The results show that the critical rainfall in 3 durations in Laoshan Datuan respectively are 62 mm， 75 mm and 95 mm in the case of Pa = 40mm. The early-warning index in the 3 durations was got by “discount” processing， ready transfer indexes are 56mm， 68mm and 86 mm， and the immediate transfer indexes are 68mm， 83mm and 105 mm.