Solid oxide cell (SOC) plays a pivotal role in the reversible conversion of electricity-gas-electricity. The seawater electrolysis through reversible SOC is proven to be an effective approach to the intermittent power generation such as offshore wind power and photovoltaic power generation. In this paper, a flat-tube SOC stack was tested to analyze the reversible performance for SOC stack fueled with seawater/H₂ under different operating conditions (i.e., water vapor content and current density). The results show that the degradation rate in electrolysis mode is minimal at a current density of 100 mA·cm^–2, a humidified temperature of 90 ℃ (volume fraction of water vapor is 65%) and 750 ℃. The minimum voltage degradation rate in fuel cell mode is attained at a current density of 100 mA/cm², a humidified temperature of 86 ℃ (volume fraction of water vapor is 56%) and 750 ℃. The efficiency of the stack in reversible mode is 95.2% at 100 mA·cm^–2. It is indicated that this study could provide a reference for the application of reversible SOC stacks in seawater environment.