As vehicle emission regulations become increasingly rigorous, the automotive industry is accelerating the development of electrified vehicle platforms such as Battery Electric Vehicles (BEV) and Plug-in Hybrid Electric Vehicles (PHEV). Since the available waste heat from these vehicles is limited, additional heat sources such as electric heaters are needed for cabin heating operation. However, using electric heaters for winter heating may consume more than 50% of vehicle’s total energy usage which results in less available energy for driving and significant reduction of the driving range of electric vehicles. Use of a heat pump is a solution to improve EV driving range at cold ambient. However prior-art heat pump systems still need the assistance of electric heaters at very cold ambient conditions and for dehumidification operations, which increases system cost and reduces the benefits of heat pump systems. In this study, an efficient gas-injection heat pump system has been developed, which achieves high cabin heating performance at low ambient temperature and dehumidification operation without the assistance of electric heaters. It uses a compact refrigerant gas-liquid separator where the outlet gas at intermediate pressure is injected back to the compressor while the outlet liquid is further expanded for evaporation. In this separator, an integrated flow regulating device based on refrigerant pressure difference is used for simple control of opening and closing the injection line. It is also used to control the system heat balance during dehumidification mode for efficient operation. In addition, dehumidification at low ambient temperature is achieved by a unique system configuration which allows parallel operation of the evaporator and the outside heat exchanger. The evaporator’s minimum pressure is limited by a pressure regulation valve to prevent icing, while heat is extracted from ambient through the outside heat exchanger to reheat the air after the evaporator.