Knowledge presentation and reasoning technique in extension theory can provide some new theories and methods for the knowledge acquisition and repository design of expert system. The extension expert system that combines extension theory and expert system has great capability in self-study and creative thinking. First, a kind of framework of the extension expert system is introduced by applying extension knowledge expressing and emanative reasoning. Then, the characteristics of the expert system are analyzed. Finally, the application of extension knowledge to the enterprise marketing forecast system as an example is given, and the general design as well as implementation scheme of the system is proposed at the same time. The example has demonstrated the feasibility of applying extension expert system to intelligent decision support system (IDSS).