From the perspective of physical mechanism of rainfall-runoff behaviour, three small watersheds in different hydrometeorological regions were selected and the design storms were calculated based on the attenuation formula of rainstorm in this study. According to the characteristics of the underlying surface of the basin, the model parameters were deduced, and the hydrological model was constructed to realize the design flood calculation. The results of using the hydrological model to estimate design floods were compared with those of the reasoning formula method. The results show: the design flood obtained by the hydrological model method is similar to that obtained by the inference formula method, with relative errors of flood peak and flood volume not exceeding 30%; the hydrological model method takes into account the variation of soil infiltration capacity during rainfall processes and the uncertainty of the catchment process at various points within the watershed; the uneven spatiotemporal distribution of hydrological model parameters is more in line with the actual runoff generation and concentration laws of the basin, improving the reliability of design flood calculation in the complex situation.