Abstract: This study investigated the method for phase control of wellbore fluid during supercritical CO₂ jet fracturing. A wellbore flow model of supercritical CO₂ jet fracturing was built and then used for example calculation and analysis. Moreover, the phase control issue was investigated in an example formation at a subnormal geothermal gradient. During supercritical CO₂ jet fracturing, wellbore pressure increases with increasing well depth, while wellbore temperature first increases and then decreases near the target zone. Wellbore pressure easily reaches the critical pressure, and the control of wellbore temperature appears to be the key factor for phase control of supercritical CO₂ jet fracturing. When the geothermal gradient is significantly low, CO₂ fluid in the wellbore of the target zone can not reach the critical temperature, thus affecting the process of supercritical CO₂ jet fracturing. In this case, increasing the temperature of CO₂ fluid injected into the wellbore will effectively promote CO₂ fluid to enter the supercritical state. This study provides a reference for phase control of wellbore fluid during supercritical CO₂ jet fracturing.