Under certain conditions, the FACTS and other fast control devices may stimulate subsynchronous oscillation problems of the power system, leading to the instability of the generator shaft, causing major accidents that endanger the safe and stable operation of the power system. As a new type of FACTS, UPFC can realize the control of bus voltage and the regulation of active power and reactive power. But the research on the effect of the subsynchronous oscillation is less. At the same time, current UPFC damping controller is more aimed at low frequency oscillation mode. So the UPFC model was established, and test signal method was used to study the effects of UPFC control parameters on subsynchronous oscillation. Finally a UPFC damping controller was designed. Computer simulations on the IEEE second standard test system shows that the controller can effectively improve the electrical damping of the multi-torsional-modes and depress the subsynchronous oscillation.