Optimizing the planting structure of grain crops is an effective means to ensure food security and mitigate greenhouse gas emissions in agriculture. Based on the compilation data of agricultural input,management,and output cost-benefit data from various provinces in China,this article uses emission factors,multi-objective optimization,and cost-benefit analysis methods to calculate the greenhouse gas emissions from grain production in China and its provinces in 2021,reveal their spatial distribution characteristics and contribution share,optimize their planting structure and layout,and carry out production cost and economic benefits analysis of major crops. The research findings are as follows: (1) In 2021,China's grain production resulted in total greenhouse emissions of 4.33 × 10⁸ tons CO₂ equivalent. CH₄ and N₂O are the main contributors,accounting for 40.12% and 32.56% of the total emissions,rice cultivation is the largest emitter,with an emission of 2.09×10⁸ tons of CO₂ equivalent,while corn is the largest emitter among dryland crops. (2) Greenhouse gas emissions from grain production exhibit a spatial distribution where emissions are higher in the eastern regions and lower in the western regions,with greater emissions in the southern areas compared to the northern areas. The rice-producing regions such as the Northeast Plain,the Mid-Lower Reaches of the Yangtze River Plain,and the Pearl River Delta are high-emission areas. (3) Following the optimization of crop planting structures,there was a 6.78% reduction in grain planting areas compared to 2021,accompanied by 4.82% and 4.87% reductions in irrigation water and fertilizer usage,respectively. The yield increased by 1.1%,with rice and wheat production decreasing by 0.62% and 0.55%,respectively,while corn production increased by 1.48%. Total greenhouse gas emissions decreased by 5.09%,with CO₂,CH₄,and N₂O emissions decreasing by 5.98%,5.6%,and 3.7%,respectively. (4) Adjusting the planting structure of grain crops can help improve cost-benefit relationships and optimize total grain crop revenue by increasing it by 305.74 billion yuan. Except for vegetable cultivation costs,which increased to some extent,all other crops decreased in cost to varying degrees. In the future,under the influence of factors such as climate change,market competition,and high production costs,the pressure on China's food security and greenhouse gas reduction will further intensify. Optimizing planting structures,strengthening agricultural industry chain cooperation,and reducing production material consumption are effective ways to achieve stable grain production and reduction of greenhouse gas emissions.