Purpose <p>In motor reducers, rotor dynamic balancing and vibration reduction have long been key challenges in the industry. This study focuses on designing a particle damper within the structure to simultaneously achieve dynamic balance and reduce mechanical vibrations.</p> Methods <p>An innovative rotor dynamic balancing technique is proposed. A particle damper was designed and mounted on the motor rotor of a reducer. Different particle weights were filled into separate chambers as a counterweight to achieve the purpose of dynamic balancing. The dynamic balancing effects of the rotor with the particle dampers were analyzed based on a two-way coupled calculation of multi-body dynamics (MBD) and discrete element method (DEM). Reaction forces at supporting points are analyzed to evaluate the effectiveness of rotor dynamic balancing. In the verification experiment of dynamic balancing, the influence coefficient method (ICM) was used to calculate different eccentricities for one set of rotors at three different rotating speeds.</p> Results <p>The dynamic balance level of the rotor system in the motor reducer could be under G 2.5. In addition, the rms value of the horizontal vibration of the reducer decreased by an average of 9.5% in the frequency range of 1000 Hz.</p> Conclusion <p>The two-way MBD-DEM coupling technology is applied to a motor reducer. The novel motor rotor with particle dampers is validated to simultaneously achieve dynamic balancing and vibration attenuation.</p>