Abstract: The model of urban drainage system unsteady flow simulation involves six blocks, namely the main program the database of storm drainage, the abstractions and losses block, the calculating inflow of sewer block, the transport block and the waterlogged areas calculating block. The model can be used for simulating sewer surcharge, calculating waterlogged areas and water depth under storm conditions. The best design of rebuilding storm sewer system can be provided based on simulating existing drainage pipeline. The model was used for rebuilding Shenyang drainage works last year, and the good result has been obtained