Once the single-phase-to-ground fault occurs in the graded power supply system, the primary substation and the secondary switching stations will select the fault line, the large number of alarm information will cause the dispatcher’s misjudge. In this paper, a single-phase-to-ground fault diagnosis technique based on rough set theory is proposed, which utilizes the data reduction capability of rough set theory to effectively integrate the relevant information, and eliminate redundant information to improve the rapidity of identification information. In addition, in the process of diagnosis, strong fault tolerance of the rough set theory is used to improve the accuracy of the diagnosis by filtering out the contradictory information and correcting the contradictory information automatically. This paper designs a practical single-phase-to-ground fault diagnosis system, which verifies the correctness of the diagnosis technology by the practical application of data analysis.