This paper adopted option game method to explore the construction cost of two - stage concession term decision model of infrastructure BOT project and concession term decision. It was found that the growth option would affect the game result both for government and project companies on the condition of maintenance cost changing over time. The author discussed the strategy solution that had showed the project companies which the growth option that would increase the investment for achieving its strategic target to enhance the lifecycle of project under the condition that the concession term comes short. At last this paper gave an example to confirm this result.