Forced reverse flow at design speed is an essential method to avoid over-temperature in centrifugal compressor when integral power unit operates as emergency power unit (EPU)mode. For reducing the compressor’s power dissipation at EPU mode, a novel variable diffuser with splitting combined vanes is adopted and the effects on reverse flow performance and characteristics of compressor are analyzed. Results show that the diffuser vane angle position has important influence on blade passage flow behavior of diffuser and impeller. Modifying the diffuser vane stagger angle can decrease the zone of flow separation and the intensity of shock, and make the flow smoothly. At fixed gas resource pressure and slot area, the reverse flow rate increases, but the power dissipation of compressor decreases as variable diffuser vane angle position is adjusted. So at the same reverse flow rate, modifying the diffuser vane stagger angle can significantly reduce the compressor’s power dissipation and enlarge the integral power unit power output.