In recent years，several policy measures enacted by China including the Opinions on Further Encouraging and Guiding Social Capital towards Medical Institutions supported the liberalization and promotion of social investment in healthcare sector. Social investment were increasingly involved in the integration，reorganization and restructuring of medical institutions and resources. According to the Implementation Rules for the Classification Management of Urban Medical Institutions，private medical institutions in China can be divided into two categories，namely non-profit medical institutions and for-profit medical institutions，which are subject to different tax，registration requirements and accounting standards. Actually，the non-profit medical institutions are more encouraged by the government and less restricted in the market access，allowing the private medical institutions often take the form of non-profit. While due to the fact there is no a clear definition of “non-profit” in law，the legality of some actions in the investment and operation of non-profit medical institutions remains questionable，especially such as how the investors receive investment returns. This paper is trying to make an analysis in respect to the legal nature of social investment in non-profit medical institutions on the basis of the existing legal system，so as to clarify some ideas on this matter.