Chillers are widely used in commercial buildings for air conditioning, and their energy consumption is the main contribution to the building’s carbon emissions. Currently, the COPs of small- and medium-capacity screw chillers are still generally lower than 6.5, whereas large-capacity commercial centrifugal chillers have achieved an ultra-high energy-efficiency level of COP ≥ 7.0. To achieve an ultra-high energy efficiency of COP ≥ 6.5 in medium-capacity chillers, the authors developed a 200 RT screw chiller by adopting the technologies of two-stage compression and interstage vapor injection. The whole development process, including the design, simulation, analysis, and experiment, is presented in this paper. It was found that the two-stage compression technology could effectively boost the performance of the chiller’s compressor to a maximum volumetric and adiabatic efficiency of 99% and 80%, respectively. With the interstage vapor injection technology, the chiller’s cooling capacity and COP were increased by more than 11% and 8%, respectively. When the use of these two technologies was combined, the maximum COP of the chiller reached 7.17. Additionally, under these working conditions, the COP and integrated part-load value (IPLV) were 6.74 and 10.04, respectively. In all, the combination use of vapor injection and two-stage compression technologies shows great potential to improve the performance of chillers. The work and conclusions described here might provide an effective reference for the future development of high-efficiency small- and medium-capacity screw chillers.