Based on the risk amplification mechanism of social psychological crisis，constructing the model of influencing factors of public security in emergency. The influencing factors are event properties，government response，media communication and group psychology and behavior. Using web crawler software to collect relevant textual data about 32 major incidents including news，Sina Weibo，Baidu search index and so on from 2014 to 2016，to explore the requirements and logical conditions leading to public security loss in significant emergencies through qualitative comparative analysis （QCA）. The result shows that large-scale influence，untimely response，existence of rumors and negative emotion are the requirements leading to the loss of public security. When other condition variables occur with the requirements simultaneously，which is most likely to trigger public security loss. Among all the influencing factors，media communication has the most explanatory power on public security，followed by event properties，group psychology and behavior，government response. Therefore，we should construct a psychological crisis management mechanism with the participation of government，media and the public in order to enhance the emergency response capability.