Through the field research on the blunt flood ditch produced after the rainstorm in research region, this article conducted the calculation of storm flood current, delimited the position of upstream runoff area. According to the flood ditch this article used the hydraulics method to estimate the section peak discharge Q, according to the theory of runoff it estimated the precipitation net rainfall (runoff coefficient α), and then used the rational formula method to calculate the peak discharge Q′ in the catchment area. When the values of Q′and Q are close, the runoff coefficient is reasonable. Then it estimated the total amount of flood. The calculation method is feasible. This method can provide reference for the similar area without data to estimate the survey heavy rains and flood.