It is the sluggish ion migration kinetics that seriously affects the practical performance of the magnesium ion batteries. Even though an electrode material design using rational interlayer engineering method could effectively solve this issue, the optimal interlayer distance remains undetermined. Herein, various VOPO₄-based electrodes with expanded interlayer spacing were fabricated and the relationship between interlayer structure and battery performance was revealed. Electrochemical analysis combined with computations unveils the existence of an optimal interlayer structure, as inadequate expansion failed to fully utilization of the material performance, while excessive expansion degraded the electrode stability. Among them, the electrode with triethylene glycol (TEG) intercalation exhibited optimized performance, maintaining excellent cycling stability (191.3 mAh·g⁻¹ after 800 cycles). Density functional theory (DFT) demonstrated the effectiveness and limitations to lowering the migration energy barrier by expanding the interlayer engineering. In addition, systematic mechanism research revealed the Mg²⁺ storage process: The stepwise shuttling of Mg²⁺ along the directions that lie in (001) plane triggers two pairs of redox processes, namely V⁵⁺/V⁴⁺ and V⁴⁺/V³⁺. This study, regulation of layer spacing to achieve the best integrated performance of electrodes, could deepen the understanding of interlayer engineering and guide the design of advanced multivalent-ion batteries.