Heterostructure engineering has emerged as a promising strategy to enhance the electrochemical CO₂ reduction reaction (CO₂RR) by optimizing interfacial electron transfer. Herein, we report a novel octahedral SnS₂/SnO₂ heterojunction catalyst synthesized via an ion-exchange vulcanization method, which achieves exceptional activity and selectivity for CO₂-to-formate conversion. Through in-situ Raman spectroscopy, ex-situ X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), we demonstrate that the octahedral SnS₂/SnO₂ heterojunction dynamically restructures into a sulfur-doped Sn/SnO₂ (Sn(S)/SnO₂) heterostructure under operating conditions. Density functional theory (DFT) calculations reveal that the Sn(S)/SnO₂ interface facilitates electron transfer from SnO₂ to metallic Sn(S), generating a built-in electric field that stabilizes Sn⁴⁺ in SnO₂ and accelerates proton-coupled electron transfer to *OCHO intermediates. Consequently, the catalyst achieves a formate Faradaic efficiency exceeding 90% over a broad potential window (−0.6 to −1.0 V vs. reversible hydrogen electrode (RHE)) with a high partial current density of −280 mA·cm⁻², surpassing most reported Sn-based catalysts. This work elucidates the structural dynamics and interfacial enhancement mechanisms of heterojunction catalysts, offering a rational design principle for advanced CO₂RR electrocatalysts.