Lead halide perovskite has attracted great attentions for the applications in perovskite solar cells and LED devices and it also has the great potential in photocatalytic applications due to high extinction coefficients, and long electron-hole diffusion lengths. However, the rapid recombination rate of photogenerated electron-hole pairs still limits the photocatalytic activity. Herein, a novel CsPbBr₃ quantum dots/S doping g-C₃N₄ ultrathin nanosheet 0D/2D heterojunctions photocatalyst are prepared by loading perovskite quantum dots onto ultrathin doped g-C₃N₄. The strategy of S element doping improved the properties of in g-C₃N₄ ultra-thin structure providing more adsorption and reaction sites for photocatalytic activity. And the type II band alignment structure of CsPbBr₃ / g-C₃N₄ heterostructure effectively improved the separation and transmission of photogenerated carriers and inhibit the recombination of photogenerated carriers, thus improving the photocatalytic CO₂ reduction performance. This study provides a facile and effective method for synthesis of halide perovskite based photocatalysts, which holds the potential for further application in environmental and energy field.