Tunnels under construction in high-geothermal environments are susceptible to fire risks. To address this issue, a 1∶20-scale experimental platform simulating high-geothermal conditions is established, with fire source location, oil pan size, and geothermal temperature as variables. The smoke stratification characteristics are investigated through scale experiments and theoretical analyses. The main conclusions are as follows: (1) In the vertical temperature profile of the geothermal area, the temperature of the lower cold air layer is primarily governed by the high-geothermal environment, exhibiting a considerable increase that becomes more pronounced with rising geothermal temperatures. In contrast, the temperature of the lower cold air layer in the non-geothermal area remains essentially consistent with the ambient temperature. (2) The theoretical model of smoke stratification proposed by Newman is applied to evaluate the stability of smoke stratification downstream of the fire source (near the tunnel portal) in high-geothermal environments. The model adopts a critical value of ΔT_cf/ΔT_avg=1.70 (with ΔT_cf being the tunnel crown-to-ground temperature difference, and ΔT_avg the average temperature rise across the tunnel section). The ΔT_cf/ΔT_avg ratio exceeds 1.70 regardless of fire source location, indicating clear downstream smoke stratification. This ratio decreases with increasing geothermal temperature, suggesting reduced smoke stratification stability. As smoke spreads toward the tunnel portal, ΔT_cf/ΔT_avg initially rises and then declines, governed by different mechanisms: in the upstream non-geothermal area, the initial rise is attributable to decreasing tunnel bottom temperature, whereas near the tunnel portal, the subsequent ΔT_cf/ΔT_avg decline is attributable to the entrainment effect. (3) The thickness of the onedimensional smoke layer increases with rising geothermal temperature.