Abstract: Fallow is a significant and effective measure to improve agricultural production capacity, promote sustainable development of agriculture, as well as drive strategy for rural revitalization in China. Since scientific determination of fallow scale is the common concern problem in the research field of fallow, it's highly urgent to study what the exact fallow scale threshold is so that scientific basis can be provided to deeply carry out the national strategy of "Storing Grain in Land" and expand fallow pilot scheme on a national scale. The superior basis condition of cultivated land has help major grain producing areas play significant roles in ensuring the national food security, while the long-term intensive utilization of cultivated land has also caused land degradation and ecological environment deterioration. By introducing the revising pressure index on cultivated land and ecological balance index, the study aimed to build a model with general applicability to estimate the largest fallow scale of major grain producing areas under both food security and ecological safety constraints, meanwhile an approach of determining the elastic boundary of fallow scale is also presented. Then Jiangsu Province was selected as the empirical area and System Dynamics model was employed to simulate and forecast the variation tendency of the largest fallow scale as well as elastic boundary of fallow scale. The results showed that: 1) during the simulation run from 2007 to 2016, the elastic boundary of fallow scale would shrink from 12.12% in 2007 to 10.16% in 2016, a certain proportion of flexible cultivated land resource can still be held in Jiangsu Province to support the implementation of fallow system when the grain distribution mission was accomplished and the ecology of cultivated land resource was also taken into consideration; 2) during the simulation run from 2017 to 2036, the cultivated land resource in Jiangsu Province would decrease to 446.63×104hm2with the rapid urbanization process and the implementation of land protection measures may have little effect to avoid the over-fast loss of cultivated land. In addition, the condition and environment of fallow would also be broken because of the deterioration of cultivated land resource ecology; 3) during the simulation run from 2017 to 2036, because the sustainability of cultivated land resource and fallow suitability index would both continue to decline over time, the maximum fallow scale and elastic boundary of fallow scale would both decrease over half at the end of the simulation, sustainable use of cultivated land appears to be less optimistic. Taking both food security and ecological safety into consideration, it was suggested to take full advantage of solid foundation of cultivated land resource in major grain producing areas, and implement normalized fallow actively and orderly. Great importance should be attached to the potential ecological security risks of cultivated land resource in major grain producing areas, the early warning index system link up with food security and ecological safety should be built to dynamically adjust the fallow scheme. Besides, the fallow system and other cultivated land protection policies, such as abandoned farmland management, land consolidation and farmland capital construction, should be aligned to make overall plans, so that positive spillover effects can be created in the internal cultivated land protection system and further promote cultivated land ecosystem through the implementation of fallow.