Based on the analysis method of emergency mechanism, this paper sorts out the key information of the evolution mechanism of the 102 unconventional emergencies occurred in 38 megacities in the world. By using DEMATEL model to search the main factors driving the evolution of the original events to sub-events and the mutual influence among them, the following conclusions are drawn: (1) During the development of unconventional emergencies in megacities, the evolutional phenomenon of events exists generally and the evolution paths are different obviously. (2) It can be seen that there is an obvious interactive feedback relationship between the emergency handling measures and event evolution in megalopolis. (3) The main department of disposal, the level of early warning and the pre-arranged planning are the most powerful factors determining the evolution of events. The research results have obvious warning significance to improving the ability of emergency management. As such, the paper is not a study of a failure in emergency cases, but rather the theoretical framework of unconventional emergencies evolution that make actions and decisions stand out as meaningful for the actors involved in dealing with situations of high complexity and uncertainty. In the future, it is necessary to study the driving results caused by various factors and explore the internal driving mechanism between the driving factors and the type of sub-events.