Under the background of the country vigorously promoting the cultural service uses PPP mode, the implementation of PPP mode in the service system of public library is the important theory and practice innovation of government and social capital cooperation. In the research, we try to probe into the relationship between public library service system and social capital's profitability; to analyze the necessity and feasibility of implementing PPP mode in public library service system, and to clarify the obstacle of the PPP mode of public library service system and the challenges in the future. The results show that the implementation of PPP mode helps to improve the management level and efficiency of public library service system, meanwhile, it helps to enhance the influence of social capital, which explains the necessity of implementing PPP mode. In addition, the good policy environment of our country provides the feasibility of implementing PPP mode in the service system of public library. However, in the process of implementing PPP mode in the service system of public library in our country, there are some obstacles that the laws and regulations   are imperfect, lacking source power and the social capital， etc. It is necessary to further explore the concrete measures to effectively solve these problems.