In this paper，the authors aim to explore the mechanim of government purchasing services based on the ideas of legitimation and empowerment along with case studies. By employing a theroetical framework of rational choice，this paper attempts to interpret how the dual mechanism of institutional trust and organizational trust play siginficant roles in influecencing the decision-making of service purchasing in the context of social changes. The authors argue that social service organizations have potentials to search possible channels to increase their legitimacy of interacting with government. On this premise，the authors discuss about how to improve the service pattern and deepen the institutions of government purchasing services.