The hidden fault characteristics of the current transformer (CT) are very weak, which affects the performance of measurement and protection equipment and easily leads to a cascading failure or even crash when the system has fault or pressure. It is necessary to make online diagnosis for timely detection of CT hidden fault and pre-warning. The fault diagnosis methods of the existing literature require feature extraction after a fault occurs, which are not applicable to the hidden fault analysis. According to the Kirchhoff's current law, based on the current constraint relationship among CT of branches in the topology of traction power supply system, CT associated matrix is provided to get the analytical value from every CT. Based on the influence law of hidden fault on the similarity coefficient between the analytical value and measured value, the voting rules are established to realize CT hidden fault online diagnosis. This method does not need additional measuring equipment. Simulation results are not affected by load current change in traction power supply system, and the selection of vote threshold is undemanding.