The purpose of this study was to reveal the effect of straw return with decomposition-promoting microbes on soil nutrients，labile organic carbon and the behind mechanism in mollisols under different nitrogen application levels.A split block design method was used to conduct the experiment on the mollisols farmland，in which no straw return and straw return with decomposition-promoting microbes were the two main factors，with four nitrogen application levels，i.e.0，135，180 and 225 kg/hm² as the side factors.Soil organic carbon，total nitrogen，total phosphorous and labile organic carbon fractions were tested.Labile organic carbon fractions were indicated by dissolved organic carbon，readily oxidation organic carbon，light-fraction organic carbon and particle organic carbon.The results showed that under different nitrogen application levels in straw return with decomposition-promoting microbes treatment，soil organic carbon，total phosphorus，light-fraction organic carbon，readily oxidation organic carbon and dissolved organic carbon contents increased by 0.61% ～ 1.55%，2.39% ～ 4.55%，2.85% ～ 45.31% 2.23% ～ 4.08% and 46.42% ～ 88.63%，respectively，while total nitrogen contents decreased by 3.74% ～ 9.55%.In no straw return treatment，when compared with 0 kg/hm²，the application of nitrogen increased soil organic carbon，total nitrogen，total phosphorus，dissolved organic carbon by 2.26% ～ 8.77%，1.65% ～ 3.30%，14.09% ～ 25.00%，4.84% ～ 21.88%，respectively，while decreased readily oxidation organic carbon by 5.26% ～ 8.83%.In straw return with decomposition-promoting microbes treatment when compared with 0 kg/hm²，after application of nitrogen，soil organic carbon and total phosphorus increased by 1.32% ～3.79% and 11.74% ～ 24.35%，respectively；while light-fraction organic carbon and readily oxidation organic carbon decreased by 12.74% ～ 21.78% and 5.71% ～ 10.46%，respectively；dissolved organic carbon increased by 57.02% under the level of N 180 kg/hm².Straw return with decomposition-promoting microbes combined with appropriate amount of nitrogen fertilizer could increase labile organic carbon fractions soil organic carbon and improve soil nutrient supply capacity，but the effect of excessive nitrogen fertilizer was not obvious.Straw return with decomposition-promoting microbes combined with N 180 kg/hm² performed the best among the four nitrogen application levels，which was expected to be a practical and effective way to improve mollisols fertility.