Analyzing the early warning of food security, this paper sets the self-sufficiency rate as the principal indicator in a standpoint of supplement. It is common to use the quantitative methods to forecast and warning the insecurity. However, this paper considers more about the probable outcome when the government intervenes. By constructing the causal feedbacks among grain supplement, demand, productive input, and the policy factors to simulate the future food security in Jiangsu province, conclusions can be drawn as the following: (1) The situation of food security is insecure if the self-sufficiency rate is under 68.3% according to the development of system inertia. (2) it is difficult to guarantee the food security in Jiangsu just depending on the increase of grain sown area. (3) The valid solution to ensure the food security in Jiangsu is to improve the productivity. Research on government decision making, scientific research departments.