Because the PPP project has the characteristics of incomplete contract, it is difficult to form a complete constraint on the private sector defaults in the PPP environment al protection project. According to strong reciprocity theory, it analyzes the public - private heterogeneity preference and the private sector opportunism tendency as the reference point, clarifies the behavior mechanism of the private sector in the PPP environmental protection project, and establishes the game model to simulate the game and The repeated game path between the public and the private. Pointing out that the government has a highly reciprocal act of altruism, while the private sector participates in the PPP project with the role of the person in person, and the government should impose severe penalties on private sector defaults., through rigorous penalties to establish a coercive relationship between the two sides, so that public-private partnership sustainable to maintain the altruism of PPP environmental projects.