In the E-commerce Service Supply Chain (ESSC), knowledge sharing among members is crucial for fast responses to the changing online market. We propose a knowledge sharing incentive model for the ESSC by incorporating knowledge complementarity and integration capacity. We develop two principal-agent based optimal incentive mechanisms for the ESSC under asymmetric information, when service providers are risk-neutral and risk-averse, respectively. Through numerical experiments, we examine in depth the impacts of the external uncertainty and risk-aversion degree on optimal incentive mechanisms. The incentive of knowledge sharing is found to be influenced by the knowledge complementarity of service providers, the knowledge integration capability of e-tailers, the risk-averse degree of service providers and the external uncertainty.