The mathematical principles and development achievements of the storm sewer discharge simulation system of Nanchang urban region designed by the authors,are elaborated in this paper.The observed precipitation and flood data are used to examine the precision of the model,and the results indicate that the absolute error is less than 10 cm in most simulations (about 62.6%),and above 30 cm in only a few simulations (2.4%).According to various stormwater flood grade simulation experiments,the forecast accuracy can reach 98% in moderate or severe flood regions,92% in gentle or light flood regions.In general,flood trend (grade) forecast is basically accurate in most of the times,but the quantitative forecast (flood depth) still has some errors,with the average relative error being about 6%.The model forecast results are basically in accordance with the actually situation,suggesting that the model can be applied in city stormwater flood forecast.In addition,the flooding processes of two key blocks in Nanchang urban region are also simulated through artificially assigning different precipitation rates,and the results show that flood starts when the precipitation rate reaches 20 mm/h,with the increasing of precipitation rate to 30 mm/h,flood becomes more severe.The draining duration is obvious different between the two blocks with different drainage conditions.In the block with poor drainage conditions,it is about 15 hrs/24 hrs required for accumulated stormwater to be completely drained off in a moderate-heavy/terrential rain process,respectively.The timing of maximum flood depth,which is 2-3 hours after the beginning of rainstorm in most blocks,also varies evidently with different precipitation rates and drainage conditions.The cause of error is also analyzed.