Straw returning is an important measure to improve the quality of cultivated land，the decomposition process of straw returning can provide nitrogen，phosphorus，potassium and other nutrients for crop growth. Understanding the decomposition and nutrient release characteristics of rice straw and wheat straw in rice wheat rotation system is the theoretical basis for formulating reasonable nutrient management system under straw returning. Through the field experiment of nylon nets method，the effects of decomposing agent on the decomposition characteristics and nutrient release of wheat straw in rice season and rice straw in wheat season were studied. The results showed that the fast decomposition period，medium decomposition period and slow decomposition period of wheat and rice straw were 0～10，10～20，20～110 and 0 ～ 10， 10 ～ 50，50 ～ 200 d，respectively； Under the action of straw decomposing agent，the time of straw decomposing rate reaching 50% and 95% was shortened by about 8 and 38 d. The cumulative decomposition rate of wheat straw reached 57% during the growth period of rice. Nitrogen and phosphorus in straw showed leaching-enrichment-release，the release amounts were 29% ～ 37% and 36% ～ 44%，respectively； While carbon and potassium of straw showed direct release and the release amounts were 55% and 92%，respectively. The cumulative decomposition rate of rice straw reached 73% during the growth period of wheat. Carbon，nitrogen，phosphorus and potassium in straw were released directly，and the release amounts were 75%，41% ～ 51%，59% and 98%，respectively. In rice season，wheat straw adsorbed and enriched nitrogen and phosphorus in soil solution during the slow decomposition period. With the gradual decomposition of cellulose and hemicellulose in straw，the enriched nitrogen and phosphorus would be released into soil； In wheat season，the release amount and rate of nitrogen and phosphorus were increased by 10.0% and 4.7%，respectively. Thermogravimetric analysis showed that straw decomposing agent mainly accelerated the decomposition of hemicellulose and cellulose of wheat straw and rice straw. In conclusion，straw decomposing agent can improve the decomposition rate and nutrient release rate of straw to a certain extent. At the same time，attention should be paid to the adsorption and enrichment process of nitrogen and phosphorus in soil solution by returning straw at the booting stage of rice. In actual production，it is appropriate to supplement nitrogen and phosphorus fertilizer at this stage，so as to avoid the yield reduction caused by nitrogen competition between straw and crops.