To investigate the carbon sequestration and emission reduction effects of mixed maize-soybean straw in field return, this study simulated straw return by microcosmic experiment, which used no straw addition as control (CK), and set two types of mixed straw and maize straw with five straw additions of 0.5, 1, 2, 4 and 10 mg/g, and then the differences in soil organic carbon (SOC) and its carbon fractions, the rate of SOC mineralization and the emission of greenhouse gases were analyzed and compared. The results showed that the addition of 0.5, 1, 2 and 4 mg/g of mixed straw and maize straw did not significantly increase the SOC content, but promoted SOC mineralization and soil CO₂ emission. The contents of SOC, microbial biomass carbon (MBC), dissolved organic carbon (DOC) and readily oxidisable organic carbon (ROC) were significantly increased when 10 mg/g of mixed straw and maize straw were added. Mixed straw treatment significantly reduced the rate of SOC mineralization, cumulative CO₂ emissions and global warming potential, while significantly increasing soil MBC content compared to maize straw treatment. However, N₂O emission rates were extremely low throughout the incubation period, with no significant differences among different treatments. In conclusion, the addition of 10 mg/g of mixed straw not only significantly promoted soil carbon sequestration, but also effectively reduced greenhouse gas emissions, which is crucial for improving the sustainability of agricultural production.