Injector in two-phase flow includes step motor, adjustable valve body, nozzle, case of injector, adaptor of injection liquid, air intake cavity, mixing chamber and pressure expansion chamber etc. In nozzle, refrigerant agent is quickened up continuously so as to reduce pressure in the air intake cavity. Thus, gaseous refrigerant agent from evaporator is injected to the air intake cavity. Then, gaseous refrigerant agent and liquid in main flow from condenser are mixed in the mixing chamber. Pressure expansion is carried out in pressure expansion chamber, and mixed flow outflows from case of injector. Mixing liquid in main flow with injection liquid is carried out under approximate equi-pressure. The pressure expansion chamber is designed based on theory of equal flow of momentum rate so as to raise efficiency of injector. Features are: few moving parts, raised efficiency of system about 5úÑ-20úÑ.