Two-dimensional transition metal borides (MBene), as emerging electrode materials for metal-ion batteries, exhibit various phase structures, including MB, M₂B, and M₂B₂. However, current research on the M₂B-phase system remains insufficient. This study focuses on the design of M₂B-phase MBenes, pioneering the construction of two novel sulfur-functionalized materials, Zr₂BS₂ and Nb₂BS₂, while systematically elucidating their performance mechanisms as anode materials for lithium/sodium-ion batteries. Through first-principles calculations, both Zr₂BS₂ and Nb₂BS₂ demonstrate exceptional structural stability and superior electrochemical properties in sodium-ion battery applications. Specifically, they exhibit high theoretical specific capacities (624 mA·h/g and 616 mA·h/g) and remarkably low diffusion energy barriers for Na⁺ (0.131 eV and 0.088 eV). Moreover, their low open-circuit voltages (0.38 V and 0.21 V) effectively suppress dendrite growth, achieving an optimal balance between high capacity and operational safety. This work not only establishes a theoretical framework for MBene-based anode design but also provides critical insights into the correlation between surface functionalization, structural stability, and ion transport kinetics. These findings provide valuable guidance for developing other two-dimensional materials and non-layered systems, while contributing to mechanistic understanding of charge-discharge processes in transition metal dichalcogenide TMD-based lithium/sodium-ion batteries.