Due to lacking of the tools to accurately evaluate the value of PPP projects, government tends to ignore the impact of various subsidy policies of the value of projects, then the concession period would be set too long which reduces the social efficiency of PPP projects. Taking highway projects as the object to study, the value of subsidy policies government offers would be analyzed and a method to assess the value of project could be given out. Causal loop diagram would be used to analyze the relevant relationships among various factors influencing concession period, stock-flow diagram would be used to build the decision-making model of concession period, and Vensim DSS would be used to simulate a practical case. The paper draws conclusions as following: the subsidy policies government offers could increase the investment value of PPP projects and then shorten the length of concession period. The subsidies consist of investment subsidy and operating subsidy, the investment subsidy has greater influence on concession period and also needs government proves more fiscal spending, the operating subsidy is closely related to quality and price and has bigger uncertainty. Therefore, when make decisions about concession period, the government should consider the influence of subsidy policies on the concession period and the fiscal risks subsidy policies bring.
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