The invention discloses a subsynchronous damping control system for effectively inhibiting subsynchronous resonance and oscillation, belonging to the technical field of stableness and control of the electrical power systems. The system comprises a rotary speed checking pre-processor, a combined mode filter, a combined proportion phase shifter, a mode control signal synthesizer, a subsynchronous compensating current command calculator, a current beat controller and a DC / AC convertor. The invention inhibits shafting torsional vibration by injecting subsynchronous / supersynchronous current complementary with machine set shafting modal frequency, and can simultaneously inhibit multiple torsional vibration modals, so that the invention has comprehensive function, can solve the multi-modal subsynchronous resonance and oscillation problems of the electrical power system, improves the supersynchronous stability of the system and lowers shafting torsional vibration fatigue loss of a high-rating generator.