The application of extension in data mining is multi-aspect. The paper discusses the method that extension data mining is applied in decision support system. The extension method enriches the content of data mining, and provides a new method for building dependent formulas. The method is also applied in the development decision-making of enterprise. Experiment result indicates that it can effectively promote farther development of decision support system. The extension data mining, which is given in this paper, is favorable to using existing data for making decision in decision support system.