Abstract: To address the engineering challenges associated with the unclear spatial distribution of the pressure arch and the difficulty in quantitatively determining its boundaries in fractured surrounding rock tunnels, this study systematically investigates the formation mechanism and spatial distribution characteristics of the surrounding rock pressure arch based on elastoplastic theory. A criterion for determining the inner and outer boundaries of the pressure arch under non-hydrostatic stress conditions is established, and theoretical formulas for calculating the span, height, and thickness of the pressure arch are derived. Combined with the engineering case of the Mamoke No. 2 Tunnel on the newly constructed Xining–Chengdu Railway, the transient Rayleigh wave method was employed to conduct field measurements of the pressure arch. Key parameters such as the boundary positions of the pressure arch, and its thickness were obtained. The results indicate that the lateral pressure coefficient has a significant influence on the distribution pattern of the pressure arch. As the lateral pressure coefficient increases, the thickness of the pressure arch at the sidewalls gradually decreases, while the thickness at the vault gradually increases. The field-measured datas are in good agreement with the theoretical predictions, with relative errors controlled within 8%, validating the feasibility and reliability of the transient Rayleigh surface wave method in identifying the boundaries of the pressure arch in surrounding rock.