The incomplete contracts, asymmetric information and credibility crisis give birth to moral hazard with hidden information and make social efficiency decrease in the construction work, According to the change of working condition for construction project, by using dynamic game theory, incomplete contract model about constructing time and price is set up. The study shows that the contract can lead to the optimal investment, conquer hidden information with moral hazard, and realize social efficiency optimization for construction work, through the determination of reasonable working condition, time, price and the rule of contract variation.