The purpose of this study was to develop a method to aid and support human operators in emergency operations. A conceptual model of system performance was constructed in order to observe the constraints of human capabilities under emergency conditions. Then the influence of emergent events and the applicable condition of procedural design principles were discussed. In order to help operators in handling the emergent event, a decision table support and a computer support were developed according to historical data and principles of procedural design. In order to verify the effects of above methods, a simulated experiment of handling the emergent event in the refrigeration system of an ammonia plant was performed. As shown by the experimental results, the support methods successfully improved the response speed of the subject, but did not improve accuracy. The results also revealed that the decision table support had a better effect on simple error type, whereas the 16-breadth computer support had a better effect on both the simple error type and complex error type.