Abstract: ：To investigate the mechanical and deformation characteristics of different arch support structures in tun? nels with weak and loose surrounding rock, the Bimoyuan Tunnel on the Leshan-Xichang Expressway was selected as the engineering background. A combined approach of field tests and numerical simulations was employed to compare the mechanical responses of four types of initial support arches after tunnel excavation: ?159×5 mm, ?140×6 mm,and 120 mm×120 mm×5 mm concrete-filled steel tube (CFST) arches, and I20b steel arches. Results show that compared with the I20b steel arches, the CFST arches reduce steel consumption by 36.2%～41.9%, shotcrete volume by15%～30%, and measured displacement by 63.08%～72.31%. However, the maximum monitored stress values were observed in CFST arches, and the range of the surrounding rock plastic zone was significantly smaller. Under all four arch support types, shear and tensile failures were more likely to occur at the sidewalls and arch feet, and the maximum vertical displacement of the arches occurred at the arch bottom, with central uplift values ranging from 11.28 mm to 11.8 mm. Therefore, CFST arches offer a favorable combination of material efficiency, high stiffness, and strong support capacity. Nevertheless, substantial plastic zones still exist at the sidewalls and arch feet. It is recommended to improve the stability of the surrounding rock by grouting reinforcement or optimizing rock bolt support parameters.