To address issues such as large deformation-induced clearance encroachment and failure of primary support structures during the excavation of Neogene mudstone tunnels, this study focuses on the Bailuyuan Tunnel in the southern section of the Xi’an Outer Ring Expressway. Through various research methods, including laboratory tests, field monitoring, and numerical simulations, the deformation characteristics of large-span highway tunnels in Neogene mudstone formations were investigated. The results indicate:Compared to Neogene mudstone formations in other regions, the Bailuyuan Tunnel’s Neogene mudstone exhibits the following distinctive features: larger interparticle porosity, weaker cohesion, higher moisture content, and softer, more fractured surrounding rock. These factors are among the primary contributors to the challenges in deformation control in the Bailuyuan Tunnel.At the same time,when the formation’s moisture content is relatively low, tunnel deformation undergoes four stages: rapid growth, sustained growth, slow growth, and eventual stabilization. During construction, the excavation of the trailing pilot tunnel significantly influences the settlement stability of the leading pilot tunnel.While,as moisture content increases, the mechanical properties of the surrounding rock gradually deteriorate, and the impact of construction methods on tunnel deformation diminishes. Under these conditions, tunnel deformation is primarily governed by the state of the surrounding rock.For large-span highway tunnels in Neogene mudstone, laboratory tests and numerical simulations were used to determine the relationship between the mechanical properties of the surrounding rock, tunnel deformation, and moisture content. The findings were validated against actual engineering cases, providing valuable construction insights for future deformation control in similar tunnels.