Abstract: Unexpected incidents involving mass participation become the important factors,influencing Chinese social stable and modernization,but there are absence of scientific understanding for its engendering mechanism and evolutionary rule.The paper builds the evolutionary game model between government and social group,and analyzes the governmental opportunism behavior that induces the unexpected incident extended.Through the evolutionary game model analysis,the main conclusions are the following:(1) If the emotion and behavior of social vulnerable group haven't been sharpened,which are remaining at the peaceful appeal under the system permission to appeal to the central authorities for help,the opportunism government will take the high-handed strong strategy.The equilibrium of game model is(peaceful appeal,highhanded).(2) If a small number of social vulnerable groups begin to take the violence rebel behaviors engaged in beating,smashing and looting,the opportunism government turns to make the concession,which induces the other people continuing to take the violence rebel behavior.So the unexpected incident will furiously extend.In the treatment of unexpected incident,the governmental opportunism behavior isn't only delaying the optimal time for controlling the incident,but also inducing the incident extended.At last,the paper analyzes the measure andperformance of emergency management of local government in the case of/Weng'an incident.

Key words: unexpected incidents involving mass participation: governmental opportunism behavior, evolutionary game:social crisis, emergency management