In order to overcome the defects in the conventional superimposed distance protection， for instance， the mal-operation resulting from an external earth fault occurring while the transmission system operates at large power angle， a novel self-adaptive superimposed distance protection scheme is proposed. By using the information provided from the wide area phasor measurement unit（PMU）， this proposed scheme can adjust the operational threshold adaptively， in responding to the real-time parameter measurements of two equivalent power systems. After discussing the various limitations of conventional solutions， the boundary conditions of mal-operation for previous criteria are deduced theoretically. A new criterion of compound superimposed distance protection is put forward in which self-adaptive threshold is combined with criterion switching. By the virtue of above arrangement， the operation reliability and sensitivity of the superimposed distance protection are improved. The effectiveness of the proposed criterion is verified by the simulation results of ATP.