Abstract: In this paper, through the comprehensive application of evolutionary game theory modeling and multi-agent social simulation experiments, we study the influence of local government opportunism behaviors in unexpected mass incidents. First, we establish the evolutionary game model between the government and the social vulnerable groups, then the results show that the local government opportunism behaviors lead to an expansive trend of emergency. Second, based on the evolution process of Kunming Luoshi Bay merchants gathered event, we use the multi-agent social simulation test method to test the analysis results of the evolutionary game theory model, and the simulation results verify the conclusions of theoretical analysis well. Therefore, the conclusion of the evolutionary game theory analysis and the social simulation results show that the local governmental opportunism behaviors not only directly lead to the expansion of the situation, but also delay the best time to handle event.