In recent years, two-dimensional (2D) materials have attracted considerable attention due to their outstanding optical and electronic properties and have shown great potential for applications in next-generation solar cells and other optoelectronic devices. In this paper, density functional theory (DFT) is applied 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, the optical absorption coefficients of these heterostructures are increased (the absorption coefficient in the x-direction reaches 10⁶ cm^-1), and they can effectively separate the photogenerated electron-hole pairs. Among 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 this structure is a direct-band gap semiconductor with a band gap of 0.86 eV (PBE) and 1.26 eV (HSE06), which can be applied 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 the 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 the range of -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 that of 2D heterojunction materials such as Cs₃Sb₂I₉/InSe (η=3.3%), SiPGaS/As (η=7.3%) and SnSe/SnS (η=9.1%). This further broadens the application scope of the BAs/I-AsP heterojunction, making it promising to play an important role in the field of photodetectors and solar cells.