Calculating the urban design rainstorm process scientifically and rationally is directly related to urban flood control safety. The paper took Nanjing as the research object, and the 1h average rainfall samples of Nanjing from 1982 to 2015 were obtained based on annual maximum method. The paper selected the optimal function from the P-Ⅲ distribution function, the Gumbel distribution function and the exponential distribution function, and the distribution of surface rainfall in the main urban area was calculated based on the 1h rainfall sample of Nanjing Station. Based on the comparative analysis of the advantages and disadvantages of typical rainstorm schedule of different duration by the same ratio method and frequency method, the same frequency method was chosen to estimate the design process of rainstorm in the main urban area of Nanjing after 12h's different return period. The results show that the P-Ⅲ curve is the best, and the rainstorm time distribution obtained by the same frequency method is more in line with the requirements of flood control and drainage engineering. The research results can provide scientific basis for the design standard of urban flood prevention.