In the increasing keen market competition, knowledge capital is gradually becoming important resource. Knowledge sharing can make organizational knowledge freely flow in definite range and save the cost of knowledge acquisition to enhance knowledge productivity and benefit for knowledge application and innovation. Itpsilas necessary for enterprise to apply fit incentive mechanism to insure the enthusiasm of staffs because of knowledge private property. The purpose of this paper is designing effective incentive mechanism of employee knowledge sharing based on the basic of Principal-agent Theory. In this paper, we assume that enterprise is principal and employee is agent, then we creates respectively incentive mathematical model of information symmetry and information asymmetry. Through the model solution, the optimal incentive mechanism contract is obtained. Finally we get some properties of optimal incentive mechanism by mathematical analysis. The results indicate that whether information is symmetrical or not, whether employee is risk neutral or not, the optimal incentive contract should satisfy that employeepsilas cost coefficient of knowledge sharing effort isnpsilat more than his output coefficient of knowledge sharing effort. The paper supplies theoretical basis and practical guide to promote staff knowledge sharing for enterprise manager.