The development of high-efficiency photocatalysts for photocatalytic hydrolysis using solar energy and water resources is of great significance for alleviating the current energy shortage. Finding rational photocatalysts remains a major challenge and great efforts have already been made. Here we propose a novel two-dimensional perovskite-based vdW heterostructure (Cs3Sb2I9/C2N) and systematically investigate its stability, electronic and optical properties and the effects of applied biaxial strain based on the first-principles approach to investigate its ability as a photocatalyst for water splitting. In order to ensure the significance of the calculation results in guiding experiments, a hybrid functional was used for all the calculations on the electronic structure. The results show that the Cs3Sb2I9/C2N heterostructure has satisfactory dynamic and thermal stability, and exhibits the characteristics of type-II band alignment in the equilibrium configuration. Charge density difference, Bader charge analysis and work function further prove that the photogenerated electrons flow from Cs3Sb2I9 to the C2N monolayer by the influence of the interface dipole, which promotes the separation and transfer of photogenerated charge carriers and inhibits the recombination of the photogenerated charge carriers. Furthermore, the Cs3Sb2I9/C2N heterostructure has a suitable redox potential for photocatalytic water splitting and exhibits enhanced light absorption in the visible light region. In addition, the electronic and optical properties of the Cs3Sb2I9/C2N heterostructure can be tuned by strain, and the Cs3Sb2I9/C2N heterostructure always possesses photocatalytic ability after applying -2% to 6% biaxial strain. These results suggest that the Cs3Sb2I9/C2N heterostructure will be a promising candidate for water splitting photocatalysts.