CsPbBr3 perovskite-based composites so far have been synthesized by postdeposition of CsPbBr3 on a parent material. However, in situ construction offers enhanced surface contact, better activity, and improved stability. Instead of applying a typical thermal condensation at highly elevated temperatures, we report for the first time CsPb(Br x Cl1-x )3/graphitic-C3N4 (CsPbX3/g-C3N4) composites synthesized by a simple and mild solvothermal route, with enhanced efficacy in visible-light-driven photocatalytic CO2 reduction. The composite exhibited a CO production rate of 28.5 μmol g-1 h-1 at an optimized loading amount of g-C3N4. This rate is about five times those of pure g-C3N4 and CsPbBr3. This work reports a new in situ approach for constructing perovskite-based heterostructure photocatalysts with enhanced light-harvesting ability and improved solar energy conversion efficiency.