In this article, the first principles calculation method is used to study the MoSi₂N₄/GeC heterostructures, and calculate its structural, electronic, and optical properties. And the effects of different biaxial strains and vertical electric fields on the band structure and optical absorption characteristics of the heterostructures are also investigated. MoSi₂N₄/GeC heterostructure is an indirect bandgap semiconductor with a bandgap of 1.25 eV, with the built-in electric field direction pointing from the GeC layer to the MoSi₂N₄ layer. In addition, its photogenerated carrier transfer mechanism conforms to the S-type heterostructures mechanism, thus improving the oxidation reduction potential of photocatalytic water decomposition, making it fully meet the requirements of photocatalytic water decomposition with pH = 0–14. Under biaxial strain, the band gap first increases and then decreases with the increase of compressive strain, and the light absorption performance in the ultraviolet region increases with compressive strain increasing. The band gap decreases as tensile strain increases, and the light absorption performance in the visible light region is enhanced in comparison with its counterpart under compressive strain. Under a vertical electric field, the band gap increases with positive electric field increasing, and decreases with negative electric field increasing. In summary, MoSi₂N₄/GeC heterostructures can be used as an efficient photocatalytic material in some fields such as optoelectronic devices and photocatalysis.