According to the features that the flood is mainly resulted from rainstorm in the southern humid area of china,the representative spate runoff and influx coefficient can be analyzed based on the observed rainfall.By using the coefficients,the annual maximum rainfall series can be converted to flood series,and the flood statistical coefficient can be computed.The case study shows that the result is very close to the observed value.The method not only avoids the assumption that the rainstorm and flood are of same frequency in mountain area,but also brings about the advantageous condition for the research on revealing the formation mechanism of rainstorm and flood.