As an important organization carrier to provide professional social work services，non-government social work agencies have developed rapidly in recent years. The emergence and development of private social work agencies have met the trend of the reform of social service，the transfer of government public service function and the government purchase of services from social agencies. The private social work agencies have played an important role in many areas，such as easing social contradictions，solving social problems，meeting the diverse service requirements of the public，realizing the specialization of social services，promoting social development. From the perspective of the interaction between state and society，this article holds that in the development of private social work agencies in mainland China，the government and non-government social work agencies have made mutual empowerment through government purchase from private social work agencies in the market，which，on the one hand，confirms the legitimacy and role of private social work agency in the social construction and therefore promotes its growth；and on the other hand，the legitimacy and the ruling foundation of the government are consolidated when the non-government social work agencies provide services which meet the requirements of the people effectively.