Abstract: As an abnormal situation, what role should play of experts in major emergencies? The existing literature pays more attention to the role of experts in the normal situation, and there is very little research on the role of expert in the abnormal situation. Based on the assumption of linear relationship between knowledge attribute and expert role, this paper establishes four ideal roles of expert in major emergencies. In reality, the ideal roles of expert are structurally constrained by factors such as government, academic community, and the public. In this way, this paper constructs the mechanism relationship between knowledge attribute, expert roles and expert system in major emergencies. On this basis, the paper puts forward suggestions for the expert system construction in major emergencies in China.