Concession period length decision is one of the core problems of Expressway BOT concession. In determining the concession period is directly related to the interests of the host government and the item company distribution, if the concession period is longer, the host country's interests; the concession period shorter will affect the interests of item company, which will lead to the failure of BOT projects. This paper analyzes some factors affecting the concession period, and analyze the effects of these factors on concession period decision-making, based on the Monte Carlo method to determine the concession period model, the theory method system which provides to judge and decision-making of BOT project concession period.