Abstract: To analyze the accuracy rate of diagnosis result and find out the cause of incorrect diagnosis for online fault diagnosis system,the function of recording historical fault data which is generated from each part of the system is developed. Using of this function,the historical fault data of fault diagnosis system for certain power grid generated during trial operation from November 2008 to February 2010 is recorded. The procedure used to analyze the operation condition of fault diagnosis system for power grid is formulated, and a statistics of whole operation information during this period is made. The cause of fault diagnosis system not starting and incorrectness of fault diagnosis results are analyzed in detail, and the improvement measures are proposed. The ideas of using historical data to online diagnose the power network fault based on historical data and its key issues are put forward, and the necessity of developing the tool for automatic maintenance for electrical secondary data is demonstrated. The achievement of this paper will play an important role in promoting the practicability of fault diagnosis for power grid.