All-inorganic halide perovskite (IHP) has been deemed promising in photocatalysis due to tunable bandgap and long lifetime of charge carriers. However, unsatisfactory photocatalytic activity and low stability prevent its practical applications. Rational construction of heterojunctions has been proved to be an efficient way to circumvent these obstacles. Herein, g-C₃N₄ nanosheet was employed to construct a 2D/2D (2D: two-dimensional) heterostructure with Cs₃Bi₂Br₉ through an electrostatic self-assembly process. Owing to the efficient transfer of photogenerated charge carriers, the activity of Cs₃Bi₂Br₉ was boosted with enhanced generation of carbon centered radicals. The optimized 10% Cs₃Bi₂Br₉/g-C₃N₄ composite displays the highest benzaldehyde formation rate of 4.53 mmol·h⁻¹·g⁻¹ under visible light, which is 41.8 and 2.3 times that of individual g-C₃N₄ and Cs₃Bi₂Br₉, respectively. The stability of Cs₃Bi₂Br₉ nanosheets and its selectivity for benzaldehyde (from 65% of Cs₃Bi₂Br₉ to 90% of the composite) was enhanced by reducing its surface energy and tuning the reaction pathway, respectively.