To clarify the mechanical response characteristics of surrounding rock convergence deformation and stress redistribution under different excavation sequences during the in-situ expansion of large-section tunnels, a research approach combining physical model tests, numerical simulations, and engineering practice was adopted. The effects of the traditional center diaphragm excavation method and the step-by-step sequential excavation method on surrounding rock convergence deformation and stress redistribution were investigated, and the stress distribution patterns and displacement deformation characteristics of the surrounding rock under the step-by-step sequential excavation method were analyzed. The research results indicate that the step-by-step sequential excavation method can effectively control the displacement and stress release rate of the surrounding rock, reducing the impact of sudden stress drops and localized deformation caused by the traditional center diaphragm method. Stress concentration characteristics were observed at the tunnel foot, with stress accumulation reaching 4.2 MPa and 4.6 MPa at the left and right tunnel foot, respectively. It was found that advanced support can control the initial deformation of the surrounding rock at the tunnel crown, whereas the sidewall rock mass on the excavation side exhibited significant displacement variations, primarily due to secondary disturbance-induced damage. The model test values, numerical simulation results, and field measurement data of surrounding rock stress and deformation during tunnel expansion were found to be in good agreement, verifying the effectiveness of the step-by-step sequential excavation method. The findings can provide data support and technical guidance for optimizing large-section tunnel excavation methods and controlling surrounding rock stability.