【Objectives】Based on the black soil area of Heilongjiang Province, this study took soybean as the research object, explored the changes of soybean yield, soil nitrogen and enzyme activity in soybean field after straw returning and nitrogen fertilizer application, analyzed the relationship among soybean yield, soil nitrogen composition and enzyme activity, and provided theoretical basis for soybean nitrogen nutrient management and black soil fertility protection in black soil area.【Methods】 In 2022 and 2023, the experiment was carried out at Daxijiang Farm of Jiusan Branch Company of Beidahuang Group. The conventional fertilization (49.2 N kg/hm2, S0N2) treatment was set up in the straw leaving field test area. In the straw returning field test area, no nitrogen fertilizer (S1N0) treatment, conventional nitrogen reduction 30% (34.4 N kg/hm2, S1N1) treatment, and conventional nitrogen application (49.2 N kg/hm2, S1N2) treatment were set up.【Results】 Compared with S0N2 treatment, the soybean yield of S1N2 treatment was significantly increased by 19.15% -24.51%, and the contents of soil total nitrogen, nitrate nitrogen, ammonium nitrogen, acid-hydrolyzed total nitrogen, acid-hydrolyzed amino acid nitrogen, acid-hydrolyzed ammonium nitrogen, acid-hydrolyzed amino sugar nitrogen, invertase activity, urease activity and protease activity were significantly increased.Under the condition of straw returning, soil total acid hydrolysable nitrogen, acid hydrolysable ammonium nitrogen, acid hydrolysable amino sugar nitrogen, acid hydrolysable amino acid nitrogen, sucrase activity, urease activity and protease activity increased first and then decreased with the increase of nitrogen application rate. Soil total nitrogen, nitrate nitrogen and ammonium nitrogen content increased with the increase of nitrogen application rate. There was a significant positive correlation between soil urease, sucrase, protease activity and soil total acid hydrolysable nitrogen, acid hydrolysable ammonium nitrogen and acid hydrolysable amino acid nitrogen content.【Conclusions】Both straw returning and nitrogen fertilizer application could increase the content of nitrogen components and nitrogen-related enzyme activities in 0-20 cm topsoil of soybean field. Among them, straw returning + conventional nitrogen reduction of 30% ( 34.4 N kg/hm2 ) was more conducive to the improvement of nitrogen supply capacity of farmland soil.