In this study, we developed a new type of water-to-water two-stage screw heat pump with a vapor injection flash tank and investigated the performance of the heat pump under various condensation temperatures of the refrigeration cycle. In addition to the flash tank, a flooded-type evaporator and a shell-tube oil cooler for refrigeration oil heat recovery are implemented in the heat pump system. Suitable vapor injection pressures corresponding to different condensation temperatures were found. It was also found in this study, that with a suitable vapor injection pressure for operation, the average coefficient of heating performance can be enhanced by the investigated heat pump system under various condensation temperatures. An intelligent controlling strategy in terms of the subcooled degree was developed to attain the dynamic high performance of the heat pump under different condensation temperatures. From the performance measurements, it can be found that the average heating capacity improvement of 29% and the performance enhancement of 11% can be achieved by the two-stage water-to-water heat pump by utilizing the controlling strategy to adjust the vapor injection and the oil heat recovery under different condensation temperatures.