For a system composed of a high-speed electric spindle and a grinding wheel, an automatic online balancing scheme is proposed to improve the balancing precision and efficiency. An automatic electromagnetic ring balancer with self-locking function is designed and developed. The balancer′s counterweight parameter, electromagnetic self-locking torque, and the shaft′s maximal angular acceleration that is accepted by the balancer are calculated. The experimental platform to evaluate the compensation capacity of the rotor′s mass unbalance is built, and the pre-balancing of the combined rotor is carried out in 1 878r/min. Based on this result, the vibration response testing of the designed balancer is also completed in 1 300r/min. The tested data show that the designed balancer is capable of reducing vibrations caused by mass imbalance below 33.77g·cm, has a total self-locking torque of 2.3546N·m, can bear a maximal shaft angular acceleration of 5010rad/s2, and is similarly effective for both the balancer′s clockwise and anticlockwise rotation.