In the rail transit traction system, the operation of the four-quadrant pulse rectifier introduces double grid frequency ripple into the DC bus voltage. This ripple voltage interacts with the fundamental frequency voltage of the inverter, leading to beat-frequency phenomena in the motor stator voltage and current. This phenomenon is particularly pronounced when the motor operating frequency is close to twice the grid frequency. This paper elaborates on the generation mechanism of the beat-frequency phenomenon, and analyzes the rotation direction and frequency of each harmonic component in the motor stator current. To address the beat-frequency problem, a beat-frequency rotating coordinate system is proposed to extract the harmonic currents causing the beat-frequency phenomenon, which are converted into DC quantities in the beat-frequency coordinate system, and the compensation voltage for beat-frequency suppression is generated by a proportional-integral controller. A motor simulation model is built in MATLAB/Simulink, and simulation verification is carried out under three different motor operating conditions. The simulation results show that the proposed method can effectively suppress the beat-frequency phenomenon in a wide frequency range where the motor frequency is close to the double grid frequency.