In recent years, two-dimensional ferroelectric heterojunctions have shown potential applications in the fields of high-density storage and optoelectronic devices. The development of novel two-dimensional ferroelectric heterojunctions is an important current research direction. In this work, first-principles calculations are used to study the band structure and stress control of In₂Se₃/InSe vertical heterojunction composed of two-dimensional ferroelectric material α-In₂Se₃ and monolayer InSe. The calculations show that the In₂Se₃/InSe heterojunction is an indirect band gap semiconductor with type-II band matching. When the polarization direction of In₂Se₃ is perpendicular to the surface facing outward, the band gap is 0.50 eV, and the top and bottom of the valence band originate from InSe and In₂Se₃ respectively; when the polarization direction of In₂Se₃ points inward the plane, the band gap decreases by 0.04 eV, and the sources of the top of the valence band and the bottom of the conduction band are interchanged. Under in-plane stretching, the greater the degree of stretching, the smaller the band gap is. After a certain threshold is exceeded, the heterojunction changes from a semiconductor into a conductor, which can also change the heterojunction with an indirect band gap into that with a direct band gap. The research results of this work show that changing the polarization direction and applying stress is an effective way to control the two-dimensional In₂Se₃/InSe ferroelectric heterojunction, which can provide a theoretical reference for designing the relevant ferroelectric devices.