<p>The mechanical performance of the bond interface in steel plate-reinforced tunnels directly governs the overall load-bearing capacity of the strengthened tunnel structure. To elucidate the mechanical behavior and failure mechanisms of the bond interface in steel plate-reinforced tunnel linings under combined tensile-shear loading, a series of static tests were conducted. The results indicate that, compared with uniaxial conditions, the interfacial bonding strength is significantly reduced under coupled tensile and shear stresses. Specifically, the tensile bonding strength decreases by up to 57.8%, while the shear bonding strength is reduced by as much as 68%. Under combined loading, the interfacial stress-strain response exhibits a pronounced three-stage evolution, comprising a linear growth stage, a rapid growth stage, and a softening stage. Variations in the tensile-to-shear stress ratio primarily influence the softening point and the ultimate stress. The dominant tensile failure modes include concrete stripping and partial debonding at the adhesive-concrete interface. In shear, three failure modes are identified: concrete stripping, debonding at the concrete-adhesive interface, and debonding at the steel plate-adhesive interface. With increasing additional stress, the failure mode progressively transitions toward concrete stripping, accompanied by an increase in stripping depth.</p>