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. The experiment 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. The study analysed the differences in soil organic carbon (SOC) and its carbon fractions, the rate of organic carbon mineralisation and the emission of greenhouse gases. 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 organic carbon mineralisation 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. The mixed straw treatment significantly reduced the rate of organic carbon mineralisation, cumulative CO2 emissions and global warming potential, while significantly increasing the soil MBC content compared to the maize straw treatment. However, N2O emission rates were extremely low throughout the incubation period, with no significant differences between treatments. The study showed that 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.