Abstract: To address the structural safety concerns of steel-shell concrete immersed tunnels under extreme fire scenarios and investigate the synergistic protective effects of fireproof boards and high-pressure water mist systems, a full-scale numerical model was established based on field test data from the Shenzhen-Zhongshan Immersed Tunnel. Systematic analyses were conducted on structural temperature responses under three configurations—unprotected, fireproof board only, and combined protection—across 50 MW, 100 MW, 200 MW, and 300 MW fire scenarios. Results demonstrated that unprotected tunnel ceilings consistently exceeded 1,000 °C, reaching 1,112 °C at 50 MW and rising to 1,369 °C at 300MW. While a single fireproof board reduced ceiling temperature to 297 °C under the 50MW fire scenario, temperatures escalated to 329 °C, 344 °C, and 353 °C at 100 MW, 200 MW, and 300 MW respectively—all surpassing the 300 °C safety threshold for steel-concrete composites. The combined protection maintained ceiling temperatures below 283 °C for fires up to 200 MW, achieving maximum cooling efficiency of 78.3%, yet reached 304 °C at 300 MW, exceeding the material's fire resistance limit. This study reveals the synergistic effect between the passive thermal insulation of the fireproof board and the active cooling and fire suppression of the fine water mist system under extreme fire conditions, which significantly enhances the fire resistance performance of the steel shell-concrete tunnel structure. The findings provides a systematic theoretical basis and design reference for the fire resistance design and extreme fire risk assessment of immersed tube tunnels.