Changing the working conditions of an industrial heat pump can cause a mismatch of the inner and outer pressure ratios of the twin-screw compressor. Over-compression and under-compression increase the compressor power consumption and decrease the heat-pump’s coefficient of performance (COP). Injecting refrigerant vapor into the compression chamber would increase the inner-pressure ratio and avoid under-compression. An experimental study was conducted with different injection pressures when the condenser outlet’s water temperature reached 90 ℃. The results show that the optimal injection pressure causes the COP and adiabatic efficiency to peak at the same time. This result shows that the vapor injection changes the compression process to a quasi-polytropic compression process. Adjusting the compressor’s inner-pressure ratio by changing the refrigerant-injection pressure is feasible.