Abstract: The code for design of outdoor wastewater engineering in China follows the storm sewer system design method of the former Soviet Union,using attenuation coefficient m for amending the catchment concentration time.In view of the method,the paper analyzes that different urban storm intensity formula parameters b, n and surface catchment concentration time t₁,flow time t₂ in pipes have impact on the design flow Q_m/Q₀ after using the attenuation coefficient m.The research finds out that Q_m/Q₀ is an increasing function of b and t₁,and a decreasing function of n and t₂.The suitable scope of urban storm intensity formula has been severely affected as a result of using the attenuation coefficient m.Depending on the different surface catchment concentration times,the suitable scope of design flow time t₂ in pipes has reduced to 57.5-52.5min.According to the urban storm intensity formula of Beijing,after using attenuation coefficient m,the design flow has reduced 40%-50%.