By contrasting the advantages and disadvantages between the passive filters and APF (active power filters), the schemeof the harmonic restraining devices on the locomotive was confirmed. The working principle of the harmonic restraining devices wasanalyzed, and detection method of the reactive component and harmonic component of the current in theory was derived, focused on theprinciple and designing method of the three-level hysteresis control strategy. The results of MATLAB simulation and experimentationshowed that the harmonic restraining devices composition of passive filter and APF on one traction winding could significantly restrainthe harmonics current and improve the current quality.