The　rational　method　(RM)　adopting　steady　uniform　flow　assumption　is　a　simple　and　mainstream　approach　for　the　design　of　urban　stormwater-drainage　systems　(USDSs).　However,　when　designing　large-scale　USDSs,　the　RM　significantly　deviates　from　the　actual　flow　regime　due　to　inappropriate　assumptions.　To　improve　the　accuracy　and　reliability　of　the　design　method,　a　dynamic-wave-simulation-based　method　(DWSBM)　is　proposed.　Firstly,　a　numerical　model　which　is　equivalent　to　the　surface　runoff　yield　process　of　RM　is　established　and　validated.　Then　the　dynamic　wave　module　is　adopted　for　pipe　flow　calculations.　This　method　integrates　hydraulic　models　for　the　whole　design　process.　Both　DWSBM　and　RM　are　used　for　USDS　design　of　two　areas,　and　design　comparison　demonstrates　that　the　design　flow　rates　computed　using　DWSBM　are　greater　than　obtained　by　RM.　With　the　increase　in　pipeline　length,　the　design　flow　differences　between　the　two　methods　in　the　two　areas　increased　from　3.09%　to　28.97%　and　from　16.01%　to　45.40%　respectively.　Adopting　the　DWSBM　for　design　flow　rate　calculation　can　effectively　improve　the　design　reliability　and　drainage　capacity　of　USDSs.