The application of CO2 two-stage compression heat pump technology with inter-stage injection significantly enhances the low-temperature performance of air-source heat pump systems. During actual operation, CO2 two-stage compression heat pumps with interstage injection often suffer from excessively high exhaust temperatures and compressor burnout due to imperfect system control strategies. This study aims to optimize the control strategy of CO2 two-stage compression heat pump systems to increase their operational reliability and practical viability. The research investigates the injection ratio (Rinj) and volume ratio (Rv), focusing on their effects on discharge temperature (Tdis,H), coefficient of performance (COP), and other performance metrics across different operating conditions. A simulation model of a CO₂ transcritical two-stage compression heat pump with inter-stage injection was developed using Dymola software to analyze the system's injection and capacity matching behavior. Subsequently, the effects of Rinj and Rv on COP and heating capacity (QH) were assessed using response surface methodology and analysis of variance. The results indicate that as Rinj increases, both COP and QH initially rise rapidly and then decline gradually, while Tdis,H initially decrease slowly, followed by a more rapid decline. At an evaporating temperature (Tₑ) of -20℃, within the rapid COP growth region, increasing Rinj from 0 to optimal injection ratio (Rinj,opt) enhances COP by 18.4% and QH by 29.1%, with Tdis,H decreasing by 3℃ for every 0.05 increment in Rinj. Beyond Rinj,opt, as Rinj approaches maximum injection ratio (Rinj,max), COP and QH decrease by 2.6% and 2.3%, respectively, while Tdis,H decreases by 7℃ per 0.05 increment in Rinj. These injection characteristics are attributed to the dynamic response of flow and heat transfer dynamics, as well as variations in intermediate pressure (Pₘ) with changing injection ratios. Furthermore, sensitivity analysis reveals that Rinj has a greater influence on COP, while Rv has a more significant effect on QH.