Two-dimensional materials have garnered significant attention due to their exceptional potential in electronic, optic and flexible nanodevices. Using first-principles calculations based on density functional theory, this paper systematically investigates the electronic structure, optical properties of the orthotropic MO₂ (M=Ge, Sn, Pb) monolayers and interface contact properties of the SnO₂/MoOCl₂ van der Waals heterojunction. It has been demonstrated that the GeO₂, SnO₂ and PbO₂ monolayers exhibit excellent dynamical, mechanical and thermodynamic stabilities, as well as in-plane anisotropic mechanical properties, and its indirect bandgap is 2.92, 2.12 and 0.95 eV, respectively. The bandgaps of the GeO₂, PbO₂ and SnO₂ monolayers decrease with the increasing of the biaxial tensile strain, while those of the GeO₂ and PbO₂ monolayers gradually increase with increasing biaxial compressive strain. The bandgaps of the GeO₂ and SnO₂ monolayers both exceed 1.23 eV under the biaxial strains, falling within the bandgap range required for photocatalytic water splitting, and demonstrate full water-splitting capability under strong acidic and neutral conditions under the compressive strains. The optical absorption peak of the MO₂ monolayers should redshift with the tensile strain, enhancing visible light absorption and facilitating water decomposition. Investigations on the contact properties of the SnO₂/MoOCl₂ van der Waals heterojunction interface reveal that it exhibits a strong n-type ohmic contact type. The findings indicate that the orthotropic lattice MO₂ semiconductors behave significant potential for the directional design of optoelectronic devices. The GeO₂ and SnO₂ monolayers should be promising candidates for photocatalysts, providing both theoretical foundations and design insights for the application of two-dimensional materials in optoelectronic devices and photocatalysis.