In recent years, two-dimensional (2D) materials have attracted considerable attention due to their outstanding optical and electronic properties, and they have shown great potential applications in next-generation solar cells and other optoelectronic devices. In this work, density functional theory (DFT) is used to systematically study the electronic and optoelectronic properties of the heterojunction formed by 2D BAs and I-AsP monolayers, as well as the response of this heterojunction under biaxial strain and electric field. The calculation results show that in the ground state, the four vertically stacked BAs/I-AsP heterostructures all have stable geometric structures, and their band gaps range from 0.63 to 0.86 eV. Compared with their constituent monolayers, these heterostructures have the increased optical absorption coefficients (the absorption coefficient in the x-direction reaches 10⁶ cm^–1), and they can effectively separate the photogenerated electron-hole pairs. Of the four structures, the A1 structure exhibits the smallest interlayer spacing, the smallest binding energy, and the highest stability. It has a type-I band alignment and a structure of a direct-band-gap semiconductor with band gaps of 0.86 eV (PBE) and 1.26 eV (HSE06), which can be used in the field of light-emitting diodes. The band gap and band type of the heterostructure can be effectively changed by applying biaxial strain and electric field. Under the application of biaxial tensile or compressive strain in a range of –10% to 8%, the band gap increases accordingly. When the tensile strain is greater than 8%, the band gap starts to decrease. When the biaxial strain ε ≤ –3% and ε > 8%, the heterojunction transitions from a type-I band alignment to a type-II band alignment. Under tensile strain, the absorption spectrum undergoes a red shift, while compressive strain leads to a blue shift of the absorption spectrum. Similarly, the externally applied electric field linearly affects the band gap of the BAs/I-AsP heterojunction in a range from –0.5 to 0.5 V/Å, and the band gap decreases as the electric field increases. When a positive electric field with E ≥ 0.2 V/Å is applied, the band alignment of the heterojunction can also transition from type-I to type-II. The BAs/I-AsP heterojunction has strong absorption properties in the ultraviolet and visible light ranges. Based on the Scharber model, the theoretical power conversion efficiency (PCE) η of the BAs/I-AsP heterojunction is found to be greater than 13%, which is higher than those of 2D heterojunction materials such as Cs₃Sb₂I₉/InSe (η = 3.3%), SiPGaS/As (η = 7.3%) and SnSe/SnS (η = 9.1%). This further expands the application scope of the BAs/I-AsP heterojunction, making it expected to play an important role in the field of photodetectors and solar cells.