Rechargeable magnesium-ion batteries (RMBs) are considered promising energy storage devices due to their high energy density, low-cost, and reliable safety. However, their development is still constrained by problems such as sluggish Mg²⁺ diffusion kinetics and poor structural stability. Herein, an urchin-like VS₄@Bi₂S₃ heterostructure cathode with moderate sulfur vacancy concentration and well-defined heterointerfaces has been successfully constructed through in-situ growing Bi₂S₃ nanorods on VS₄ microspheres via rapid (25 min) microwave-assisted solvothermal method. The appropriate sulfur vacancies provide abundant active sites while mitigating structural degradation caused by excessive defects. Besides, the material forms a Type-II heterojunction via V-S-Bi interfacial chemical bridging, inducing a built-in electric field that significantly enhances electron transport, facilitates Mg²⁺ adsorption (−1.90 eV) and diffusion (energy barrier of 0.47 eV), and buffers volume changes during cycling. Electrochemical evaluations demonstrate that the optimized VS₄@Bi₂S₃ electrode delivers an initial discharge capacity of 1407.69 mAh·g⁻¹ and stabilized at about 353.50 mAh·g⁻¹ at 0.05 A·g⁻¹, which is significantly higher than that of VS₄ (227.31 mAh·g⁻¹) and Bi₂S₃ (152.29 mAh·g⁻¹). A combination of ex-situ/in-situ characterization and theoretical simulations reveals the synergistic magnesium storage mechanism involving intercalation and nanoconfined conversion reactions, along with interfacial dynamic stabilization. This work offers new insights into rational material design and mechanistic understanding for developing high energy density and long-life RMBs.