<p>In this work, 304 stainless steel/high-manganese steel clad plates with varying thickness ratios were successfully fabricated using a hot rolling bonding process, and the effects of thickness ratio on the microstructural evolution, elemental diffusion behavior, and mechanical properties of the bonding interface were systematically investigated. Under a hot rolling reduction of 75%, sound metallurgical bonding was achieved in all combinations, characterized by stable austenite on both sides of the interface. The results indicate that increasing the thickness ratio intensified plastic deformation in the interfacial region, leading to significant grain refinement, an increased fraction of low-angle grain boundaries, and elevated dislocation density. Concurrently, a higher thickness ratio facilitated the cross-interface gradient diffusion of key alloying elements (Al, Cr, and Ni) while effectively suppressing the growth of the interfacial intermetallic compound (IMC) layer. Driven by the evolution of microstructure and interfacial states, the tensile strength of the clad plates exhibited an upward trend with increasing thickness ratio. In contrast, elongation decreased, aligning with the dominant mechanisms of matrix strengthening and dislocation strengthening. Conversely, the interfacial shear strength was controlled by a competitive mechanism between enhanced metallurgical bonding and interfacial work hardening embrittlement, displaying a non-monotonic trend of initially increasing and subsequently decreasing. This reveals the nonlinear regulatory effect of the thickness ratio on the interfacial toughness of the clad plates.</p>