van der Waals heterostructures are widely used in the field of photocatalysis due to the fact that their properties can be regulatedviaan external electric field, strain engineering, interface rotation, alloying, doping,etc.to promote the capacity of discrete photogenerated carriers. Herein, we fabricated an innovative heterostructure by piling monolayer GaN on isolated WSe2. Subsequently, a first principles calculation based on density functional theory was performed to verify the two-dimensional GaN/WSe2heterostructure and explore its interface stability, electronic property, carrier mobility and photocatalytic performance. The results demonstrated that the GaN/WSe2heterostructure has a direct Z-type band arrangement and possesses a bandgap of 1.66 eV. The built-in electric field is caused by the transfer of positive charge between the WSe2layers to the GaN layer, directly leading to the segregation of photogenerated electron–hole pairs. The GaN/WSe2heterostructure has high carrier mobility, which is conducive to the transmission of photogenerated carriers. Furthermore, the Gibbs free energy changes to a negative value and declines continuously during the water splitting reaction into oxygen without supplementary overpotential in a neural environment, satisfying the thermodynamic demands of water splitting. These findings verify the enhanced photocatalytic water splitting under visible light and can be used as the theoretical basis for the practical application of GaN/WSe2heterostructures.