Two-dimensional transition metal borides (MBene), as emerging electrode materials for metal-ion batteries, exhibit diverse phase structures including MB, M²B, and M²B². However, current research remains insufficient in exploring the M²B-phase system. 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^-1 and 616 mA h g^-1) 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. The findings offer 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.