Since the broader the range of speed regulation and the higher the power density of traction motor was needed, the vibration noise was increased. Therefor aiming at the generation mechanism of electromagnetic vibration noise of traction motor, the identification method of characteristic frequency was proposed suitable for different electromagnetic vibration noise source. The magnetic field harmonic of motor stator and rotor was analyzed, thus the mathematical analytical expression on field frequency is elicited. Then the degree and frequency of electromagnetic excitation force was researched, thereby the characteristic frequency of different electromagnetic vibration noise source was summed up. Through the diagnostic analysis on the testing data of electromagnetic vibration noise, the accuracy of diagnostic method proposed was verified.