Abstract: The denitrification and decomposition (DNDC) model simulates carbon and nitrogen cycles on the basis of biogeochemical processes, and it has been widely used to simulate greenhouse gas emissions in rice (Oryza sativa) paddy fields. However, few studies have used the DNDC model to simulate the effects of green manure on paddy fields in southern China. In this study, we applied four management scenarios, including a control (no N fertilizer, no green manure), N fertilizer (200 kg/ha), alfalfa (Medicago sativa)+N (3000 kg DM/ha+200 kg N/ha), and broad bean (Vicia faba)+N (3000 kg DM/ha+200 kg N/ha), to investigate the effects of green manure on rice production and greenhouse gas emissions. The overall aim of the study was to establish the relationships between green manures and production and greenhouse gas emissions by using the DNDC model. The results showed that the average grain yields in the two years were 41.85%, 29.81%, and 25.36% higher in the alfalfa+N, broad bean+N, and N fertilizer treatments, respectively, than in the control. The most pronounced increase in CH₄ emissions was in the broad bean+N treatment, which had a high C/N. The greenhouse gas intensity (GHGI) was not significantly different between the alfalfa+N, control, and N-fertilizer scenarios. Through adjusting the cropping parameters in the DNDC model, the simulated values and observed values for grain yield were quite similar, and the R² value between them in a correlation analysis was 0.89 (relative mean deviation, -0.8%). Air temperature, CO₂ concentration, soil organic carbon, and the soil clay fraction were all sensitive to CH₄ and N₂O emissions. Temperature, CO₂ concentration, and soil organic carbon were all positively related to CH₄ and N₂O emissions, while the soil clay fraction was negatively related to CH₄ emissions. These results indicated that the localized DNDC model could accurately simulate the effects of alfalfa green manure on rice grain yield and greenhouse gas emissions.