The urban small watershed is located at rapid development urbanization area.The peak discharge of design flood is an important basis for determining the urban flood control safety. The basic principle of synthetic unit hydrograph, reasoning formula, city hydrology and drainage formula method used widely in design flood calculation of urban small watershed is respectively stated, and the peak discharges of design flood in Minzhi river basin of Shenzhen are respectively calculated by using the above four methods. The results show that although the results obtained by different methods have certain difference, but the relative error of the results between synthetic unit hydrograph method and other methods is within the allowable range. The result obtained by the synthetic unit hydrograph method as the design result can meet the safety of urban flood control.