It is necessary to get the certain standard of design storm values for urban drainage planning.The specialty of urban design storm is that the statistical distribution has an anti-S shape,the design standard is lower and the duration is short.The frequency computation by two-end limited lognormal distribution and generalized exponential distribution as the type of frequency curve is presented in the paper.Through the practical example,it is found that the computation is convenient and the good curve-fitting results will be reached by these two distributions.