Power system intelligent alarm is mainly concentrated on the rule reasoning domain. This method does not effectively use the analysis of historical failure to analyze and handle the accident which is currently happening in the system. In this paper, after a preliminary study on fault diagnosis algorithm with case-based reasoning, a matching algorithm is proposed based on vector calculation. In this algorithm, the fault information in the system is abstracted and the similarity to the cases within system case library is then calculated to try to match the fault information to the case information one by one. All the history cases are obtained by manual analysis of historical failure information in the system case library. The fault properties and the cause of similar cases are helpful for current failure analysis, diagnosis and exception handling.