The invention relates to a dynamic stabilization device and a method for subsynchronous oscillation of a large generator set. The device comprises a speed measuring cabinet, a controller, a three-phase TCR and a step-down transformer; the speed measuring cabinet is connected with the generator set and outputs deviation values of speed signals of the generator set to the controller; the controller comprises a signal processing unit and a main control unit, the controller is connected with the three-phase TCR, and the signal processing unit comprises a filtering unit, a phase correction unit, a scale amplifying unit and an arithmetic unit; an output end of the three-phase TCR is connected with the step-down transformer; and the step-down transformer is connected with a high-voltage bus in a power grid. The device and the method fill the technical gap of inhibiting the subsynchronous oscillation by utilizing SVC technology and have the advantages of convenient control, simple structure, low cost and the like in terms of inhibiting the subsynchronous oscillation. When a plurality of generator sets exist on site, the speed signals of a plurality of machines can be acquired by expanding the signal processing unit of the controller.