Encouraging employees to share tacit knowledge among organizations is critical to a firm’s success. Tacit knowledge propagates through direct contact among individuals. This process exhibits characteristics that are similar to those of spreading epidemic between individuals. The paper constructs a dynamics model of tacit knowledge transmission to describe the tacit knowledge spreading in two organizations with employee mobility applying the modeling thought of the infectious diseases transmission dynamic method. The paper also considers the introduction and departure of employees. A threshold that governs whether tacit knowledge can be shared among organizations exists. Numerical simulations are presented to support the aforementioned theoretical results. The paper confirms, from both theoretical and quantitative perspective, that encouraging employee mobility appropriately can effectively promote tacit knowledge sharing among employees between two organizations. Further, if employee mobility is unreasonable, it will play an inhibiting effect.