We proposed a vapor injection dynamic control method for a two-stage compression heat pump system with a circulating water heating method in order to attain high operational performance with increasing condensation temperature of the heat pump. During the water heating process, the circulating water is heated from a normal temperature of 35 ℃ to a high temperature of 80 ℃ and the corresponding condensation temperature of the refrigeration cycle is also increased from 40 to 85 ℃. Owing to the dynamical increases in the circulating water temperature and condensation temperature of the refrigeration cycle, the automatic and optimal control of the heat pump system in operation cannot be implemented through the traditional feedback control method by directly sensing the vapor injection pressure of the refrigeration cycle. It is found that, for optimal performance of the developed system in dynamical operation, the vapor injection pressure of the heat pump under each condensation temperature is related to the corresponding subcooling degree. The subcooling degree of the refrigeration cycle was obtained from the temperature sensors in the condenser and at the refrigerant inlet of the expansion valve. In this study, regarding the optimal performance operation of the heat pump system, a regression model of the subcooling degree in terms of different condensation temperatures of the refrigeration cycle was built in order to control the vapor injection pressure with the dynamic variations of the condensation temperature of the refrigeration cycle. By sensing the condensation temperature of the refrigeration cycle, the corresponding subcooling degree of the refrigeration cycle for the optimal vapor injection pressure can be determined from the regression model. Finally, the openness of the control valve on the vapor injection piping route was automatically adjusted to approach the corresponding optimal subcooling degrees of different condensation temperatures to attain high operational performance. The experimental results are in agreement with the theoretical results within the range of 10–20%. In comparison with the system with constant vapor injection pressure, the coefficent of heating perfromance (COPh) of the system can be increased by 20% with the vapor injection pressure control. From the experimental results, it is found that COPh and the required power of the heat pump system using the proposed automatic control method are in reasonable agreement with those obtained by the optimal adjustment at different condensation temperatures. This indicates that the proposed model of automatic vapor injection can be applied to dynamically control the intermediate pressure of the refrigeration cycle to maintain high operational performance of a two-stage compression heat pump.