Abstract: The catenary isolating switch is an important equipment in the traction power supply system, and its reliability is of great significance for stable operation of the whole power supply system. The isolating switch faults have the characteristics of variability, randomness. It is difficult to make a prompt and effective diagnosis on those faults. In this paper, according to the mechanical characteristics of isolating switch operation, it was deduced the function relation between the motor current and the output torque, found the quadratic function relationship between them, proved that the motor current could be used to characterize the running state of the isolation switch. A certain amount of motor current was obtained by the motor current acquisition system, the feature of motor current was extracted by fractal dimension theorem. The identification and classification of the mechanical faults of the isolating switch were implemented in a certain extent. It provided a new idea for the fault identification of catenary isolating switch.