Abstract: In order to solve problem of fault diagnosis of fault indicator for distribution power network in mining area in imperfect information, the concept of the minimum fault diagnosis area was put forward. The fault diagnosis mathematical model of fault indicator based on the concept of the minimum fault diagnosis area was proposed, and a fault diagnosis method of fault indicator based on the worst principle and the mathematical model was proposed. The method recognizes and corrects short circuit fault signals and the fault grounding signal through the worst principle, and solves fault diagnosis problem such as false negatives and false positives of fault signal in imperfect information. The running example proves the practicability and correctness of the method.