The invention provides a flexible direct current additional subsynchronous oscillation control system for suppressing subsynchronous resonance and oscillation. The system comprises a feedback signal processor, a multi-mode band-pass filter, a multi-mode proportional phase shifter, a mode control signal synthesizer and an additional signal controller. A subsynchronous damping control strategy is added on a flexible direct current control system, a plurality of torsional vibration modes can be suppressed at the same time, functions are comprehensive, multi-mode subsynchronous resonance and oscillation of a power system can be achieved, subsynchronous stability of the system is improved, transmission capacity of a power grid is further improved, and safety of a generator is ensured.