All-inorganic metal halide perovskites represented by cesium lead chloride have become important candidates for the development of high-performance photovoltaic and optoelectronic devices due to their excellent optoelectronic properties and defect tolerance. However, poor structural stability has become a bottleneck for its commercial applications. In this work, we propose to integrate thin layers of bismuth oxychloride (BiOCl) on the surface of cesium lead chloride perovskite (CsPbCl₃) to form a van der Waals heterojunction. And we systematically study the environmental stability of BiOCl/CsPbCl₃ van der Waals heterojunction and the influence of interfacial effects on its optoelectronic properties by combining first-principles calculations and ab initio molecular dynamics simulations. The calculated results show that the van der Waals integrated BiOCl on the surface of CsPbCl₃ can greatly improve its environmental stability, which is due to the highly stable BiOCl layer isolating the reaction of water and oxygen molecules with the perovskite lattice. Moreover, the two BiOCl/CsPbCl₃ van der Waals heterojunctions show a type-II band structure, which conduces to promoting the carrier separation. At the same time, the two heterojunctions have small effective carrier mass, which well preserves the excellent carrier transport properties of CsPbCl₃ and BiOCl. However, CsCl-terminated heterojunctions exhibit larger band orders than PbCl₂-terminated heterojunctions, which can lead to higher open-circuit voltages and lower dark currents in CsCl-terminated heterojunctions. Owing to the different band gaps of BiOCl and CsPbCl₃, the heterojunctions show high optical absorption coefficients in the visible-to-ultraviolet region. This work provides a new idea and theoretical basis for improving the structural stability of CsPbCl₃ perovskite materials and their applications in high-performance optoelectronic devices.