Abstract: The main body responsible for the social security system consists of enterprises, employees and the government, and the choice made by each side based on its own interests will directly affect the coverage of social insurance. On the basis of the tripartite game model, this paper tries to analyze the game behaviors of enterprises, employees and the government; then it studies the motivation of their conspiracy to escape participation in the social security system. The result shows that the probability of their conspiracy to escape participation in the social security system is closely related to the corporate profitability, the payment rate of social insurance, the benefit level of social security, and government supervision cost. When the payment level of social security in an area is beyond the affordability of the enterprises and the earnings of the employees from the social insurance, the enterprises, employees and the local government in that region will have less motivation to participate. Then the probability of their conspiracy to evade social security payments will rise, which cannot be solved by relying simply on strengthening the collection.

Key words: tripartite game; collusion; Nash equilibrium; social security