In order to satisfy the demand of on-line automatic balance of a grinding wheel and motor spindle, we introduced an on-line automatic balance system and designed an automatic electromagnetic balancer. We conducted a study on the design method of interference fits in an automatic electromagnetic balancer based on the finite element method. The simulation results demonstrated that the interference tolerance fit for this automatic electromagnetic balancer was between 4μm and 16μm. We made automatic electromagnetic balancers with an interference tolerance of 5μm, which satisfied the demand of on-line automatic balance. Finally, we conducted experiments to test the function of the automatic electromagnetic balancer at 1 834 r/min. The experimental results showed that the unbalance-induced vibration could decrease by at least 80%.