中文摘要：

多年来频发的重大突发事件证明传统的应急决策模式已不适用。基于此，认为重大突发事件应急决策的重要内容之一就是即兴决策。以逐步推进与对比分析的方式为主线，总结了重大事件、突发事件与重大突发事件的特征并对重大突发事件进行了界定；依据决策理论对一般决策、应急决策与即兴决策的涵义和主要特征进行了研究和归纳，给出了重大突发事件即兴决策的定义。最后还分析了在危害程度不同的事件、事件发生的不同阶段、不同的决策层级等条件下即兴决策应用的频率和重要程度。

英文摘要：

Facts for the past many years have proved that the traditional decision making under emergency does not apply under serious emergencies. This paper points out that improvised decision-making is one of the important contents during serious emergency decision-making. It defines improvised decision-making under serious emergency as a mode with steps and comparisons, and summarizes the properties of important event, emergency. The connotation and main characteristics of general decision-making, emergency decision-making and improvised decision-making are discussed based on the decision-making theory. Furthermore, the improvised decision-making on serious emergency is defined. Finally, it analyses application frequency, importance and the impact of improvisational decision-making in different events with varying hazard, different stages in the emergency and different levels of decision makers.