Abstract: To investigate the deformation characteristics of shield tunnels in soft soil with pre-reinforcement under surface surcharge, a 1:5 scaled model test and three-dimensional finite element numerical simulation were conducted, based on a metro tunnel project in the Pearl River Delta. By comparing the tunnel deformation, structural internal forces, and surrounding rock responses between the pre-reinforced (using triaxial mixing piles) and unreinforced conditions, the coupled deformation mechanism of shield tunnels in soft soil under surface surcharge was revealed. The experimental results indicate that under surcharge loading, the horizontal and vertical displacements of the pre-reinforced tunnel were reduced by 31.3% and 22.7%, respectively, compared to the unreinforced group. Furthermore, the development of tunnel ovality was significantly delayed, and the failure load capacity was increased by 58.3%. Numerical simulations further demonstrated that the composite stratum formed by the triaxial mixing pile pre-reinforcement effectively disperses additional stresses. The vertical extent of the reinforcement zone significantly influences the distribution of tunnel internal forces and the stress concentration in the surrounding rock. Specifically, when the reinforcement extends to 3 m above the tunnel crown, the failure ovality is reduced by 26.21%. The study confirms that pre-reinforcement measures can significantly enhance the deformation resistance of shield tunnels in soft soil; however, the reinforcement scope should be optimized by considering the potential damage risk at the reinforcement interface.