For a heat pump, three injection styles including liquid, two-phase, and vapor injection are used to improve its performance. But there is no quantitative index to define the injection styles. They are distinguished from the injection configurations. The subcooler refrigerant injection (SCRI) is generally considered as the vapor injection. In this study, the injection super-heating degree (Delta T-inj) is introduced as a technical parameter to define the injection style. The Delta T-inj is affected by the subcooling degree (Delta T-sc). These two parameters are used to study their effects on the heat pump performance. Experimental and theoretical analysis reveals that three injection styles can be selectively achieved in the SCRI configuration by the suitable control of the main and subcooling electronic expansion valves. Additionally, for liquid and two-phase injection, it can improve heating capacity (Q(h)) by heightening the Delta T-sc to increase the heat exchange amount in the evaporator. For vapor injection, in addition to increase of the Delta T-sc, the Q(h) can be enhanced by raising the Delta T-inj to lift the compression power of the compressor. The two-phase injection is more beneficial to enhance the heat pump performance than the liquid injection and two-phase injection. The largest Q(h) and corresponding increase amplitude (delta Q(h)) were, respectively, obtained with two-phase injection to be 13 577 W and 13.5%, and the corresponding COP and increase amplitude (delta COP) were, respectively, 2.36 and 16.8%.