The invention discloses a technical method used for producing an emergency preplan against an unexpected environmental accident. The technical method of the invention includes the establishment of a case database of unexpected environmental accidents, the representation and organization of an unexpected environmental accident case, a method for searching the unexpected environmental accident case and a method for matching the unexpected environmental accident case. The 'emergency preplan system of unexpected environmental accidents based on reasoning by cases' comprises five parts, namely, a human-machine interface, a search engine, an adaptor, a case manager and a case database, and can not only 'quickly and correctly' produce an emergency preplan of the unexpected environmental accident, but also have a function of active learning. The technical method and the system can help strengthen the abilities of rescue organization and fast response in an unexpected environmental accident to greatly reduce loss of life and property for people, and have important economic values and social significance.