Abstract: This study aimed to evaluate the effects of the nitrogen fertilizer optimization on the greenhouse gas emissions in the wheat/maize fields in the Huang-Huai-Hai Plain. There was the better balance in the crop yield and environmental sustainability. The conventional nitrogen application was set as the control group. The experimental groups included the reduced nitrogen application, organic fertilizer replacement, and inhibitors regulation. Meta analysis was finally implemented to clarify the impact of the nutrient fertilizer optimization on the greenhouse gas emissions in the wheat/maize fields. The results showed that nitrogen reduction could decrease the CO₂ emissions by 10.32% and N₂O emissions by 29.34% in the wheat/maize fields, compared with the conventional practices. There was the nonlinear positive correlation with the CO₂ reduction and nitrogen reduction ratios, indicating the greater efficacy in soils with pH > 6.5. N₂O reduction demonstrated the linear correlations with nitrogen reduction ratio. The optimal performance was achieved in soils with the organic matter content of ≤20 g/kg and nitrogen input >300 kg/hm². A nitrogen reduction of less than 20% was identified as the optimal threshold, in order to balance the yield stability and environmental benefits. There was the great contribution to the influencing factors on the greenhouse gas emissions under the reduced nitrogen fertilization. The soil pH shared the greatest impact on the CO₂ emissions, accounting for 30.5%. The contributions rates of the annual average precipitation and soil organic matter to the CO₂ emissions under reduced nitrogen fertilization were 19.8% and 17.8%, respectively. The soil SOM content had the greatest impact on the N₂O emissions, accounting for 24.5%. The nitrogen application rate was the second most important factor, contributing 20.8%. Organic replacement increased the CO₂ emissions by 14.26%, but reduced N₂O emissions by 15.97% in the wheat/maize fields. CO₂ emissions linearly increased with the replacement ratios, particularly under the conditions of the annual temperature ≤13 ℃, organic matter content of≤10 g/kg, and nitrogen input >300 kg/hm². Conversely, N₂O reduction was correlated positively with the substitution ratios. The maximum efficacy was achieved in loam soils with the annual precipitation ≤800 mm, temperature ≤13 ℃, and nitrogen input ≤200 kg/hm². A replacement ratio less than 30% was optimized from the emission reduction and yield enhancement. There was also the great contribution to the greenhouse gas emissions under the organic fertilizer replacement. The soil SOM content had the greatest impact on the CO₂ emissions, reaching 58.6%. The annual average temperature and crop type were contributed 24.4% and 22.6% to the N₂O emissions, respectively. Inhibitor regulation was significantly reduced the N₂O emissions by 42.84% in the wheat/maize fields. The N₂O emission reduction and production were ranked in the descending order of the nitrification inhibitor > combined application of urease/nitrification inhibitor > urease inhibitor. The best effect of 3,4-dimethylpyrazole phosphate was found among the nitrification inhibitors. There was the contribution rate of the influencing factors on the greenhouse gas emissions under the control of inhibitors. The type of the inhibitors had the highest contribution to the N₂O emissions, at 26.5%. While the contribution rates of the nitrogen fertilizer application rate and soil organic matter content were both above 15.0%. The finding can also provide a strong reference to optimize the nutrient measures in the grain fields, in order to balance the high crop yield, soil fertility, and greenhouse gas emission.