[Purpose/significance] Exploring the constraints on knowledge contribution activities and their acting mechanism, will provide professional virtual community the empirical supports for future development tasks. [Method/process] The constraints obtained from open interviews are extracted through partial procedures of grounded theory methodology, and acting mechanism of constraints are analyzed by means of interpretative structural modeling and actor network theory. [Result/conclusion] Eighteen constraints on knowledge contribution activity form a complex dynamic actor network composed of multi-level hierarchical structure as a whole, involving five types of actors: user psychology and state, user experience, system function and design, information quality and operation mechanism. The professional virtual community needs to grasp the priorities of development tasks, and first solve key issues such as information quality and operation mechanism.