How to effectively manage straw returning and nitrogen (N) application in order to develop the straw-returning technology with efficient N saving is an urgent problem to be solved in agriculture production of Northeast China. With a field positioning experiment, the effects of different straw returning methods (no returning; crushed-returning and ploughing, S; decomposed-returning and rotary tillage, DS) and N application rates (N 180, 210 and 240 kg/hm²) on soil microbial biomass carbon/nitrogen (MBC/MBN) and the key enzyme activities of N metabolism were investigated. The results showed that compared with no returning, soil MBC and MBN contents, MBC/MBN ratio and urease activity in all S and DS treatments increased significantly, but nitrate reductase activity changed irregularly. With the extension of maize growth period, soil MBC and MBN contents for S and DN treatments showed single peak curve and double peak curve respectively, and urease and nitrate reductase activities both changed with a fluctuating trend, and both reached peak points at spring maize exuberant growing period (jointing stage to filling stage). With increase of N application rate, both soil MBC and MBN contents for all S and DS treatments increased, MBC/MBN ratio decreased, and the changes of urease and nitrate reductase activities differed from returning methods. Under the condition of the same N application rate, S treatment with a 15% postponing of N fertilizer could increase soil MBC and MBN contents, and decreas MBC/MBN ratio. In conclusion, the technology of crushed returning ploughing with N 210 kg/hm²and a 15% postponing of N fertilizer has the potential to improve N management and enhance soil fertility in the agricultural producing areas of Northeast China.