Based on the inter-area oscillation characteristics in interconnected power system， the machines participating in oscillation can be equivalent to a two-machine system according to their respective inertia center. The electromagnetic torque coefficient for the equivalent two-machine system is firstly deduced， which lays foundations for the electromagnetic torque analysis of inter-area oscillation mode. On this basis a wide-area power system stabilizer（PSS） with two-channel feedback structure is presented， and the extended phase compensation method is adopted to coordinate mapping and conversion by a transfer function between wide-area and local feedback state variables. At last the design method of the parameters for the wide-area PSS is provided systematically. Simulation results show that it can not only damp inter-area modes effectively， but also improve small signal stability limit. Compared with other stabilizer design methods， the approach proposed is helpful to improve the damping characteristics of the system.