An active electromagnetic damping was adopted to suppress the unbalanc ed vibration of rotor online. Based on analysis of the online balancing principl e of electromagnetic damping, a damping rotor test- rig was established to inves tigate the active balancing function of electromagnetic damping. Synchronizing el -ectromagnetic forces were obtained by controlling the gain currents of two sets coils to compensate the original imbalance of the rotor body. The experimental r esults indicate that, by using the optimized magnitude and the phase of electrom agne tic force, it is possible to reduce the vibration peak value of No.2 bearing in horizontal and vertical directions about 69％ and 55.6％, respectively, and the vibration peak value of the four measuring points is lower than 10 μm after the automatic balancing process. From this example, it can be concluded that the me thod can effectively control the vibration induced by th e rotor unbalance without shutdown process.