Abstract: According to the altitude compensation capability of the plug nozzle and the actual flow characteristics on plug surface,the thrust of plug nozzle was divided into two parts.Two different models of the pressure distribution on plug surface were established to analyze the change of nozzle efficiency with altitude and the effect of pressure distribution on the plug surface.Results show that the thrust aroused from the altitude compensation capability is the cause of the non-monotone variation of nozzle efficiency with altitude at high ambient pressure; the maximum of the thrust efficiency occurs before the design point at low ambient pressure.