Abstract: The nature of the external problems of PPP system is a state of imbalanced interests. From the subjective angle, it presents a positive externality at the three levels，i.e., the government department, the private sector and the public; from the objective angle, it has a negative externality at the other three levels, i.e., the speculativeness of the public investment, the dual character of the scale economy and the uncertainty of risk-transferring. Viewing the dilemma of PPP from the external perspective, it can be seen that the core of the PPP positive external problems is the attributes of public interests being restricted by the constraint conditions of the bounded rationality of the subject; while the core of the negative external problems is the attributes of the incomplete contracts being still restricted by the constraint conditions of the opportunistic behavior trending. However, there exist realistic disagreements among the development volume, the institutional supply and the practice effect of present China’s PPP system; this development plight stems from the mismatch between the institutional supply and the demand. The economic law should give full play to its motility of responsive law, so as to optimize the constraint conditions of the external problems of PPP through the competitive transformation of pluralist good-governance. In order to enrich and perfect a flexible and suitable soft law governance mechanism, an effective incentive and supervision mechanism should be formed that is oriented with different externalities. And a coordinate balance mechanism with different law branches should be used comprehensively.

Key words: public-private partnerships; externality; balance of interests; constraint condition