Abstract: Returning stover to the field can significantly improve the physicochemical properties of the soil, which is a main way to utilize stover resources and is of great significance for conserving black soil in Northeast China. Addressing the problems of high carbon and nitrogen ratio, low water content, and slow decomposition of the stover returned to the field, the returned stover to the field by mixing biogas slurry was put forward in this paper by making full use of biogas slurry rich in water, ammonia nitrogen, and microorganisms, as well as combining the characteristics of late fall and early spring in cold regions where there is a significant difference in temperature between day and night and the stover is frequently in the process of freezing and thawing. The decomposition characteristics of stover mixed with biogas slurry under freezing-thawing conditions, as well as their effects on the nutrients and microbial communities of black soil, were systematically studied under two conditions: fixed freeze-thaw temperature and natural outdoor freeze-thaw temperature. The results showed that the mixed biogas slurry significantly increased the decomposition rate of the returned stover. The decomposition rate of the stover showed a tendency to increase and then decrease with the increase of the biogas slurry. The freezing and thawing temperatures, along with the number of freeze-thaw cycles during the initial freezing-thawing phase, as well as the temperature during the middle and later stages of continuous decomposition, significantly influenced the decomposition rate of the returned stover. Both biogas slurry and maize stover contributed to replenishing organic carbon and essential nutrients such as nitrogen, phosphorus, and potassium in black soil. The enhancement effect was more pronounced when biogas slurry and maize stover were applied together. When the amounts of biogas slurry and corn stover were 96 mL and 7.7 g, respectively, corresponding to a mass ratio of biogas slurry to stover of 12.5 g/g, the decomposition rates of stover after 108 days reached 64.73%. Meanwhile, comparing with the pure corn stover blank control without biogas slurry, the organic carbon, total nitrogen, available phosphorus, and available potassium contents in each soil layer within the 0-20 cm effective plow layer increased by 11.42%-23.75%, 4.14%-21.95%, 13.06%-21.46%, and 21.33%-42.41%, respectively. The microbial communities in the black soil after returning the stover with mixing biogas slurry shifted from “oligotrophic dominance” to “eutrophic dominance”, mainly included the phyla of Proteobacteria, Acidobacteriota, Bacteroidota, and Firmicutes, as well as the genera of Brevundimonas, Pedobacter, and RB41, etc., with decomposition of organic matter, heterotrophic nitrification, and nitrogen fixation. The findings of this study provide a foundation for promoting stover return technology and the effective utilization of biogas slurry in northern cold regions.