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　Qm/Q0　after　using　the　attenuation　coefficient　m.　The　research　finds　out　that　Qm/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　57.5-52.5　min.　According　to　the　urban　storm　intensity　formula　of　Beijing,　after　using　attenuation　coefficient　m,　the　design　flow　has　reduced　40%-50%.