Abstract: The popularity of the Internet makes eWord-of-mouth (eWOM) as an important means of corporate marketing and grasping eWOM diffusion mechanism can help enterprises develop marketing strategy more effectively. In view of the process of eWOM diffusion is a multi-factor interactive process, this paper establishes eWOM diffusion system dynamics model and explores eWOM diffusion mechanism from two aspects: eWOM diffusion main body and eWOM diffusion motivations. The results show that in the whole eWOM diffusion, the amount of information received grows faster in the first half and the rate of self absorption is faster in the latter half for eWOM receivers, while eWOM senders always play an important role. Furthermore, by improving eWOM senders’ professionalism and reliability, eWOM receivers’ involvement, as well as the motivations of eWOM diffusion, the diffusion of eWOM can be promoted significantly.