This work investigates the transient performance and stability of CO₂/H₂O co-electrolysis in an air-free environment using a flat-tube solid oxide electrolysis cell (SOEC) stack. The results showed that the transient behavior of the stack with and without blowing gas into the air electrode is almost the same. With a current density of 0.67A·cm^–2 @750 °C, the stack operated for over 200 h under co-electrolysis conditions without air blowing, and the voltage drop rate of the stack was approximately 0.203%/100 hours. Microstructure analysis revealed a significant loss of nickel particles and an apparent formation of an insulating phase strontium chromate (SrCrO₄) on the surface of the current collection layer of the air electrode, which are identified as key factors contributing to the performance degradation of the stack. This study provides a reference for development of efficient fuel preparation technology based on SOEC stack in airless environments.