Abstract: A reasonable concession period design is an important prerequisite for the success of public privatepartnership(PPP) rail transit projects. To address the problem that a fixed concession period is difficult to handle the uncertainty while effectively motivating social capital, a flexible concession period model for PPP rail transit projects considering a reasonable buffer is proposed based on critical chain project management. First, the basic operating period of the project is clarified by coupling the net present value method and demonstration project data. Second, 24 uncertain factors affecting the concession period are identified, and an exploratory factor analysis is used to obtain six common factors and their influence on the concession period weights. Third, according to the dynamic characteristics of the influence of various factors on the concession period varies  with the operating stage, the initial buffer model of the flexible concession period of PPP rail transit projects is constructed using the shear and paste method and variable weight ideology. Finally, the dynamic adjustment coefficient of buffer zone is proposed according to the operational performance, and the buffer zone is dynamically managed through the reward and punishment mechanism. The results, verified by examples, show that the flexible concession period considering reasonable buffer is more aligned with the practical needs of PPP rail transit projects than the fixed concession period and can more effectively motivate social capital to continuously improve operational performance.

Key words: public private-partnership rail transit project, flexible concession period, critical chain project management, reasonable buffer, dynamic management