Due to the specific national conditions and the imperfect PPP regulations，there are some problems with the subject qualification of social capital（private sector）in PPP model in China compared with which in major western countries. On the one hand，the “social capital” in major western countries are mainly private enterprises，while the state-owned enterprises have become the main forces of social capitals in China；on the other hand，there are several special cases where financial institutions or the priority investors of funds are selected as social capitals in government procurement in practice. These phenomena are related to the organizational forms and the nature of ownership and other formal standards of social capital，as well as the core capabilities and responsibilities and other substantive standards of social capital. This article attempts to answer these hot questions from the perspectives of legal hermeneutics and hypothesis of rational man of western economics.