In order to find out reasons for local government illegally occupying cultivated land and reduce such acts to ensure China’s grain security, this paper made an analysis through building the single person game model for local government’s legal or illegal occupation of cultivation land. It reached the conclusion that the benefit obtained by local government from illegal occupation of cultivated land is far greater than cost and punishment of such illegal acts. Illegal act is an optimal choice of local government to maximize its benefits, which is also the major reason for its illegal occupation of cultivated land. Thus, to reduce these illegal acts of local government, it should reduce benefits of local government obtained from illegal occupation of cultivated land, and increase costs for illegal occupation of cultivated land.