The purpose of this study is to establish an intelligent expert system for nuclear power plant emergency response. A new framework of environmental risk management methodology by the concept of pattern recognition was introduced in this paper. A knowledge-based decision support system for emergency response and risk management of nuclear power plant was also discussed. The mathematical pattern relationship of accidental release effects on neighboring area and the corresponding response measures were presented in this paper. With this decision system, the decision maker can specify the procedure and minimize their human error in the decision process. The improvement of risk response and the quality of management system could be upgraded by this system. Besides, the methodology can also be served as a basis for the future development of environmental risk response system design.