Abstract: 【Objective】After prolonged shutdown, supercritical CO₂ pipelines often contain CO₂ in a gas–liquid two-phase state, making direct restart a significant safety risk. Due to engineering constraints, the pipeline inlet station is equipped only with compressors, and systematic experimental studies on the restart process under gas-phase CO₂ injection conditions are currently lacking. Therefore, this study aims to reveal the coupled variations in temperature and pressure and the phase evolution path of CO₂ in the pipeline during gas-phase injection, quantify the effects of initial liquid holdup and injection flow rate on restart time and CO₂ consumption, and establish a theoretical prediction formula for restart time applicable to these conditions.【Methods】A fully transparent high-pressure CO₂ phase behavior evaluation apparatus was designed and constructed. Gas-phase CO₂ injection restart experiments were conducted with initial liquid holdups of 0.6, 0.7, and 0.8 and injection flow rates of 25 and 45 mL/min. A high-speed camera system was used to record the phase transition process in real time, while temperature and pressure data were synchronously collected. Based on mass conservation and the Peng–Robinson equation of state, a theoretical prediction formula for complete liquefaction time was derived and further corrected by considering variations in relevant parameters during the injection process.【Results】The results show that under gas-phase CO₂ injection, CO₂ undergoes a phase transition path of “gas–liquid two-phase → liquid phase → dense phase”, while the pressure rise exhibits three characteristic stages: stable, slowly rising, and sharply rising. As the initial liquid holdup increases from 0.6 to 0.8, the complete liquefaction time decreases from 1,098 s to 155 s, corresponding to a reduction of approximately 86%, while CO₂ consumption decreases from 457.5 mL to 64.6 mL. At an initial liquid holdup of 0.6, increasing the injection flow rate from 25 to 45 mL/min reduces the complete liquefaction time by approximately 49.5% and CO₂ consumption by approximately 9.2%; however, an excessively high injection flow rate accelerates the pressure rise and may introduce safety risks. The mean absolute percentage error (MAPE) between the predicted and experimental values decreases from 8.02% for the theoretical formula to 5.79% for the corrected formula.【Conclusions】Gas-phase CO₂ injection enables the gradual liquefaction and slow pressurization of CO₂ from an initial gas–liquid two-phase state. The initial state and injection flow rate are important factors affecting the complete liquefaction process. The proposed prediction model provides a theoretical basis for estimating the time required for gas-phase CO₂ injection restart of supercritical CO₂ pipelines.