Low-speed permanent magnet synchronous motor (Low-speed PMSM) which used in torque wrench calibrating device usually works below 300 revolutions per minute. There was a lot of high order harmonic in the motor phase current due to the distortion of air-gap magnetic field, the dead-time and tube voltage drop of the inverter, which caused the torque ripple and the output torque fluctuates during loading, the measurement process was influenced. A novel harmonic suppression method for low-speed PMSM based on voltage compensation was proposed in view of this problem. The model of low-speed PMSM was established considering the existence of current harmonics. The method of voltage compensation was adopted, and harmonic compensation was calculated based on the model of low-speed PMSM. The current harmonics were compensated by PI control strategy. Simulation results show that the method based on voltage compensation can suppress the harmonic currents and reduce torque ripple effectively.