Abstract: Abstract: Sustainability in the grain production system is closely related to the entire food processing and supply chain for the national food security. It is a high demand to explore the sustainability of the Grain for Green Project for the grain production system, in order to accurately assess the relationship between ecological and food security. In this study, a comprehensive evaluation index system was constructed for the sustainability of the grain production system using the relevant data of grain production in various counties and districts of the Loess Plateau in 2000, 2010, and 2018. A typical ecological restoration project "Grain for Green" was then analyzed to clarify the impact of returning farmland on the sustainability of the grain production system. The results show that: 1) The sustainability of the grain production system on the Loess Plateau in 2018 was higher than that in 2000, indicating a gradually increasing trend. Specifically, the sustainability of the grain production system was dominated by the supply of natural resources, and then shifted to a stage dominated by the sustainable socio-economic development. 2) There were small differences in the sustainability of grain production systems on the Loess Plateau, most of which were in the low-to-medium sustainability level in 2000. Furthermore, the sustainable growth rate of the grain production system on the Loess Plateau reached 31.03% and 28.81%, respectively, from 2000 to 2010 and from 2010 to 2018. Therefore, the period from 2000 to 2010 presented a substantial improvement in the sustainability of the grain production system. 3) Although the cultivated land was greatly reduced (-33.48%) under the background of the Grain for Green Project, the average altitude and slope also showed a downward trend (decreased by 20.3 m and 0.88°, respectively), and the soil erosion modulus decreased from 16.14 to 9.34 t/(hm2·a), while the water volume and grain production potential increased by 5.09 mm/km2 and 53.75 kg/(hm2·a) annually, respectively. 4) The sustainability of the grain production system exhibited significant improvement under the background of the Grain for Green Project, indicating a positive effect of project implementation on the sustainability of the grain production system in the six sub-regions. More importantly, there was an increase from 0.14 to 0.17 in the sustainability index of the grain production system in the rocky mountainous area, which was the most affected by the project of the Grain for Green. Correspondingly, the Grain for Green Project can return the farmland to the ecologically fragile areas, particularly of soil erosion and land desertification. Therefore, the spatial distribution pattern greatly varied in the quality, average altitude, and height of cultivated land, indicating the reduced material supply of food production. As a result, the soil erosion modulus was reduced to effectively improve the water conservation capacity, the ecological environment, and the sustainability of the grain production system. On the whole, the Grain for Green Project has promoted the sustainability of the grain production system for the better ecological restoration of national food security.