Abstract: Stainless steel structures have gradually found application in underground engineering projects such as highway tunnels, utility tunnels, and tunnel reinforcement due to their corrosion resistance and low maintenance costs. However, the mechanical properties of stainless steel structures deteriorate severely under high temperatures during fires. To investigate the fire resistance of stainless steel in depth, a high-temperature mechanical behavior test system for stainless steel structures with aqueous soil layers was designed and developed. The system was used to reveal the spatio-temporal evolution patterns of temperature in stainless steel specimens and the surrounding soil. Based on the experimental data, formulas describing the temperature variation over time for both soil and stainless steel at any initial soil water content were established, and a method for determining the effective thermally affected thickness of the soil layer was proposed. Results indicate: (1) The temporal trends of temperature field and strain field in stainless steel specimens are consistent, though the strain response lags behind the temperature change by approximately 7 minutes. (2) A higher soil water content can effectively delay the degradation of mechanical properties in stainless steel, reduce the accumulation rate of thermal strain, and postpone the time of yield and failure. (3) Under the electric heating conditions of this test, soil water content decreases approximately linearly over time at high temperatures. (4) Based on calculations with simplified formulas, a method for determining the effective thermally affected thickness of soil layers is proposed, which can provide a reference for the fire resistance design of stainless steel tunnel structures.