Constructing van der Waals (vdW) heterostructures has emerged as an effective strategy for enriching the physical properties of two-dimensional materials and optimizing their optoelectronic performance. In this work, we systematically investigate the electronic properties and biaxial strain modulation of Janus MoSSe/g-C₃N₄ heterostructures with two distinct interfacial configurations—SMoSe/g-C₃N₄ and SeMoS/g-C₃N₄—by means of first-principles simulations. Binding energy comparisons and AIMD simulations are performed to determine the most stable stacking pattern of each type of the heterostructure. The analyses of the electrostatic potential and work function reveal that the intrinsic dipole of MoSSe layer and the interfacial electric field in the SMoSe/g-C₃N₄ heterostructure undergo a constructive superposition. This enhances the overall built-in electric field, which points from g-C₃N₄ layer to MoSSe layer, resulting in a type-I band alignment. In contrast, in the SeMoS/g-C₃N₄ configuration, the two fields oppose each other, leading to a net electric field directed from MoSSe to g-C₃N₄ layer. This leads to a type-II band alignment, which facilitates spatial carrier separation and significantly enhances photocatalytic water-splitting activity. Furthermore, this study also demonstrates that biaxial strain can effectively modulate the electronic band structures of both types of heterostructures. In particular, the SeMoS/g-C₃N₄ system exhibits a reversible transition between type-I and type-II band alignments under specific compressive (–4%) and tensile (+5%) strain states. The underlying mechanism is elucidated by the difference charge density calculations. This study provides theoretical insights into the role of interfacial and intrinsic dipoles combined with strain engineering, offering a viable route for designing efficient MoSSe/g-C₃N₄-based photocatalysts and optoelectronic devices.