Considering the characteristics of construction projects，a multi-task incentive model between the owner and contractor was established based on the principal-agent theory． An extended form of Cobb-Douglas production function was built to reflect the outcome of construction projects in the circumstances of multi-dimensional efforts invested in schedule，cost and quality． Ｒesults indicate that the optimal amount of effort devoted to each of the three tasks is positively associated with the total factor productivity，and negatively associated with the marginal changing rate of cost-of-effort． Besides，the optimal incentive coefficient is influenced by two kinds of factors: one is related to the contractor，including the total factor productivity，absolute risk-aversion coefficient，and the marginal changing rate of cost-of-effort; the other is related to the significance of each task and the uncertainty of the circumstances． Our research results could help the owner select appropriate incentive intensity and achieve reasonable risk sharings．