Stacking two-dimensional materials into heterogeneous structures is an effective strategy to regulate their physical properties and enrich their applications in modern nanoelectronics. The electronic structure and optical properties of a new two-dimensional Janus Ga₂SeTe/In₂Se₃ heterojunction with four stacked configurations are investigated by first principles calculations. The heterojunction of the four configurations is an indirect band-gap semiconductor with a type-II band structure, and the photoelectron donor and acceptor materials are determined by the polarization direction of two-dimensional In₂Se₃. The light absorption rises to 25% in the visible region, which is conducive to the effective utilization of the solar visible light. The biaxial strain can induce direct-indirect bandgap transition, and the applied electric field can effectively regulate the bandgap of heterogeneous structure. The bandgap of AA2 configuration increases monotonically from 0.195 eV to 0.714 eV, but that of AB2 configuration decreases monotonically from 0.859 eV to 0.058 eV. The band of the heterojunction always maintains the type-II structure under the two kinds of configurations. The heterojunctions under compressive strain show better light absorption capability in the visible region with shorter wavelength. These results reveal the regulatory mechanism of the Janus Ga₂SeTe/In₂Se₃ van der Waals heterojunction electronic structure and provide theoretical guidance in designing novel optoelectronic devices.