Cs3Bi2Br9 has emerged as a competitive lead-free photocatalyst owing to its low toxicity and favorable stability. However, its practical application is still limited by the rapid recombination of photogenerated carriers and a narrow photoresponse range. Herein, we report a facile in situ photodeposition strategy to engineer Cs3Bi2Br9 with controlled silver loadings, enabling the selective construction of S-scheme Cs3Bi2Br9@AgBr and Type I Cs3Bi2Br9@Cs2AgBiBr6 heterojunctions. Among these, the optimized S-scheme heterojunction (CBB-Ag2) delivers a CO yield 4.7 times higher than that of pristine Cs3Bi2Br9, achieving superior CO2 photoreduction performance. Mechanistic investigations reveal that the built-in electric field within the S-scheme heterojunction establishes efficient interfacial charge transfer channels, effectively suppressing carrier recombination and promoting •OH generation, which collectively lower the energy barrier for *COOH formation—the rate-determining step in CO2-to-CO conversion. This work presents a scalable and phase-controllable strategy for constructing high-performance, lead-free perovskite photocatalysts, offering valuable insights into rational heterojunction design for efficient CO2 valorization toward carbon neutrality.