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The manner of straw returning is a double-edged sword for cropland ecosystems because it can not only return nutrients for crop,enhance the soil fertility and lessen the use of chemical manures correspondingly,but also it may result in nonpoint source pollutants such as N,P and COD,which were released to water system in the process of its decomposition.How huge is the potential effect of the nonpoint source pollution when straws were applied to paddy fields? In this research,the simulated experiment under submerging condition was conducted to investigate the direct releasing characteristic of nonpoint source pollutants involving N,P and COD in the process of two methods about returned strew to soil such as surface cover(SC) and strew burying(SB).The results taking a period of 38 days showed that under submerging condition,the concentration of TN in surface water was decreased continuously,while TP,COD was increased urgently or slowly and then decreased continuously,respectively.The releasing intensity of TN was higher than that of TP,while the releasing speeds of TN and COD decreased continuously,except that the releasing speed of TP increased slightly at the beginning of submerging.The concentration of nonpoint source pollutants in the covering water of SC was more than that of SB,and it was increasing with the application amount.From the perspective of controlling the agricultural nonpoint source pollution,the appropriate time for TN prevention was in the initial stage of submerging,while the time for the TP and COD prevention was in 1—3 weeks after submerging.The strew burying was the friendly method.The above conclusion could provide the scientific basis for determining the negative effect of straw manure reasonably and limiting the nonpoint source pollution resulted from the returned straws to soil.