PPP项目贷款契约的不完全性会导致项目公司在费用风险发生时和银行进行再谈判，此时再谈判的结果及项目融资的有限追索权属性一方面可能诱发项目公司在事前选择危险项目策略（资产替代），另一方面，则可能出现项目公司无法获得银行追加贷款从而导致项目提前中止的情形。针对以政府付费为回报机制的PPP项目，运用不完全契约理论构建贷款契约模型，分析无政府担保情形下再谈判对资产替代和项目提前中止的影响机制，进而分析政府担保对事前（项目公司和银行再谈判前）和事后（项目公司和银行再谈判后）项目效率的影响，最后讨论同时引入政府担保和契约保证金制度的效果。研究结果表明：在无政府担保情形下，通过竞争招标不能同时抑制资产替代和项目中止问题；政府担保虽然能够避免事后项目中止实现事后效率，但无法抑制事前资产替代问题；同时引入政府担保和契约保证金制度可以保证同时实现PPP项目的事前和事后社会效率，并且能够提高政府的财务效率。研究结果为政府决策提供了理论支持，以期为实践中的PPP项目实施提供指导意义。
In recent years, public-private partnerships (PPPs) have been widely applied to infrastructure projects in China. Due to the great financial risks of PPP projects, the government often provides guarantees for these projects to attract private participation. However, although the guarantee reduces the cost of the debt, it may also lead to opportunistic behaviors on the part of the private company. Focusing on the renegotiation between project company and the bank when project risk occurs, the debt contract for a PPP project is formulated in the form of an incomplete contract model and the impacts of government guarantee on both ex-ante (before the renegotiation) and ex-post (after the renegotiation) efficiencies of the project are analyzed. Ex-ante efficiency loss caused by asset substitution may arise from incompleteness of debt contract and limited recourse of project financing. Meanwhile, ex-post efficiency loss arises from project termination, when the project company has failed to meet its debt obligations and cannot obtain additional loans from the bank. First, basic model investigates conditions that trigger the ex-ante asset substitution and ex-post project liquidation in the incomplete debt contract, via which the project company determined by a competitive bidding raises project funds. Based on the basic model, how the government guarantee influences the ex-ante and ex-post efficiencies of the project in government guarantee model is analyzed. Furthermore, contractual deposit system is introduced to improve the effects of government guarantee. The results indicate that competitive bidding cannot prevent the occurrence of both asset substitution and project termination. Although government guarantees can avoid ex-post project termination, they cannot prevent asset substitution. Therefore, the government should take the tradeoff between ex-ante efficiency loss and ex-post efficiency improvement into account when adopting the government guarantee. Finally, it is found the contract deposit system is complementary to the government guarantee to improve the social efficiency as well as the financial efficiency. The results obtained in this paper provide a theoretical support for the implementation of PPP projects.
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