To improve the consistency of rainfall runoff calculation methods in urban small catchments, based on the hydrological basic theory of urban small catchments, the similarities and differences between the rational formula method and the SWMM motion wave method for calculating surface runoff were analyzed. A volume-based runoff coefficient calculation method involving soil type, design rainfall intensity, and impermeable area ratio was proposed. A case study was carried out in Beijing. The proposed method was used to calculate the volume-based runoff coefficient, which was taken into the rational formula to calculate the peak flow of rainstorm runoff, and compared with the calculation results of SWMM. The results indicate that under the assumption of uniform spatiotemporal distribution of rainfall and hydrological losses, the rational formula method is a special case of the motion wave method. The concept of volume-based runoff coefficient is consistent with that of rainfall runoff coefficient, and is numerically equal to that of flow runoff coefficient. Based on the volume-based runoff coefficient, the peak flow of rainstorm runoff in Beijing calculated by the rational formula has a high consistency with the SWMM calculation results.