【Objective】To solve the problems of soil organic carbon deficiency in the subsoil layer and the yield of crop straw being too large in volume to make full use of in farmlands of black soil in Northeast China, a long-term (2015—2018) field experiment was carried out. 【Method】The field experiment was designed to have three treatments, i.e. CK (no straw returned), FS (returning of chopped straw) and KL (returning of pelletized straw), and three straw returning rates, i.e. Rate 1 (15 t•hm^-2), Rate 2 (45 t•hm^-2, 3 times as high as Rate 1) and 3 (75 t•hm^-2, 5 times as high as Rate 1). The straw was deeply incorporated into the subsoil layer. Soil samples were collected from all the plots separately each year for analysis of soil organic carbon content, organic carbon structure and soil nutrient ratio in an attempt to find out a way to turn waste into treasure and hence to promote sustainable development of black soil. 【Result】Results show: 1. Organic carbon content in the subsoil of the treatment plots varied in the range of 2%-20%(1 a), 5%-27%(2 a) and 1%-18%(3 a). The effect of straw returning raising organic carbon content in the subsoil was significant and improved with increasing returning rate and the effect of the treatments the highest in returning rate were the most significant and particularly in the second year, which indicates that organic carbon accumulation occurred mainly in the second year. Two-way variance analysis shows that straw returning rate was an important factor affecting soil organic carbon; 2. Nuclear magnetic resonance test shows that at the end of the experiment, the organic carbon in Treatment FS5 was found the highest in aliphaticity, while that in Treatment KL5 the highest in aromaticity, indicating that chopped straw is more likely to promote formation of organic carbon of alkyl carbon chain, while pelletized straw is to promote formation of organic carbon of aromatic hydrocarbon type; 3. In Treatments FS5 and KL5, soil C/N and C/P ratios increased by >10%, and C/K ratio did by >20%. The effect in Treatments KL was short and instant, while that in Treatments FS was long and slow. 【Conclusion】In a conclusion, deep incorporation of processed straw at a high rate can significantly increase organic carbon in the subsoil layer and help keep soil carbon in balance with nitrogen, phosphorus and potassium. It is, therefore, a feasible way to build up the black soil in thickness and fertility and to solve the difficult problem of straw returning in Northeast China.