In China, achieving sustainable development goals for PPP projects is an urgent issue. Project governance mechanisms are important to PPP project sustainability, but there are many risk factors in traditional project governance mechanisms. This research focuses on embedding risk governance mechanisms into the traditional project governance system with contract governance mechanisms and relationship governance mechanisms. The main purpose is to reveal the integration of PPP project governance mechanisms and its impact on governance performance and project sustainability, and to propose governance optimization strategies. First, by enriching the understanding of the governance mechanism, governance performance and project sustainability, and improving the measurement scale. Then, by constructing a structural equation model, collecting data through questionnaires, and exploring the effect of the project governance mechanism on project sustainability. (1) The results of direct effect testing show that contractual governance mechanisms, relational governance mechanisms and risk governance mechanisms are positively associated with governance performance, and governance performance is positively associated with project sustainability; (2) the results of mediating effect testing show that interaction of the three governance mechanisms has a positive effect on governance performance. The research results provide a new approach and perspectives for improving project governance mechanisms and achieving sustainable development in the practice of PPP projects.