A new control strategy for superconducting magnetic energy storage（SMES） is presented. The control strategy fully utilizes the flexibility of SMES in exchanging active and reactive power with power systems in four quadrants to control tie-line power flow. The objective is to synchronize centre of inertial of sending and receiving subsystems under transient conditions and quickly damp post-fault inter-area oscillation. Back-stepping method is used to derive control law for active power output of SMES. Proportional control is used for reactive power output of SMES. The designed controller is robust to model errors and bounded external disturbances. Computer test results on a four-machine two-area system demonstrate that the presented controller can effectively damp inter-area oscillation of power systems.