Abstract: The characteristics,the sources and the mechanism of the urban rainfall runoff pollution are introduced in this paper,and it is helpful for the studies in our country.The urban rainfall runoff pollution originates from the precipitation,the urban surface and the drainage system.The urban surface and drainage system are the main sources of the urban rainfall runoff pollution.The sediment and the associated pollutant deposits,forming in the combined sewer networks,are a major source of pollution during a storm.Several studies show that the erosion of the in-sewer pollutant stocks can provide 20-60%(depending on site and rain event characteristics) of the total SS,COD,BOD₅.The initial period of the stormwater runoff,in which the concentration of pollutants is substantially higher than in the later periods,is called the fist flush phenomenon.During the first flush,an enormous quantity of pollutants are discharged into the receiving waters.The research concludes that the the first flush results in a substantial concentration peak at the beginning of storm events.However,the concentration peak may vary with the different pollutants during the same storm event,or with the same watershed during different storm events.In general, the first flush is influenced by many parameters,such as the watershed,the rainfall intensity,the impervious area,and the antecedent dry weather period.Therefore,it is difficult to establish the universal regression curves,or even the complex multiple regressional functions,which can predict the first flush loads.