Based on reasoning formula, according to the flash floods rainfall warning requirement, the study removed the rectangular generalized of watershed area distribution in some concentration flow from the original formula, and made an inversion to obtain the critical rainfall calculation formula for the various typical durations, and then used isochrones method to solve the quantitative problem of the most concentration flow area in the calculation formula. The proposed calculation method of the rainfall warning has a clear physical concept, is easy to understand and master, is convenient for calculation, and is unnecessary to use hydrological data, which can be used as a new method for analysis and calculation of the flash flood warning rainfall.