Abstract: With the rapid development of infrastructure projects in western China, the demand for expressway tunnelling has been increasing. Among them, ultra-large cross-section tunnels present considerable challenges for construction due to their low flatness ratio, large span, and complex excavation procedures. This study takes the Lihuashan Tunnel, a key control project on the Tianfu New Area-Qionglai Expressway, as the research object, and systematically investigates construction methods suitable for ultra-large cross-section tunnels. Numerical simulations were conducted to compare the differences between the double side drift method and the six-step steel frame rock wall method in terms of surrounding rock stability and structural stress characteristics, thereby verifying the safety and reliability of the latter. Further optimization of construction sequences for different steel frame rock wall method construction schemes was carried out to determine the most effective sequence. Based on field monitoring data, the deformation characteristics of surrounding rock and the stress characteristics of primary support structures during actual construction were analyzed, further validating the engineering applicability of the six-step steel frame rock wall method. The results indicate that this method can ensure construction safety while significantly improving progress.