The method of generator voltage control based on wide-area information is proposed for on-line damping low frequency oscillations. Based on the mechanism for the effect of generator voltage control on oscillation damping in power system, the effects of local and inter-area oscillation modes on voltage control are analyzed. The methods of damping low frequency oscillations, which include the selection of control points based on participation factors of the generator dominant mode, the calculation of the generators mean phase angle through separating the oscillation generators into two parts, to define the larger mean phase angle of units as advance cluster and to raise the advance cluster voltage, and the identification of local and inter-area oscillation modes based on the generator dispatching area attribute, are proved effectively and correctly by the results of China Southern power grid.