In this paper, we systematically designed vdW heterostructures by stacking the GaN and Janus-WSSe monolayer and chose the structure that has potential to be used as photocatalysts of water splitting for further investigation. By applying a biaxial strain, the electronic, optical and photocatalytic properties of this heterostructure were investigated to explore whether it is suitable for photocatalyst of water splitting. Results show the heterostructure show a type II band alignment under a − 2% strain, and the band gap is 2.26 eV which is ft to absorb solar light. The band edge of it straddles the water redox potentials at pH=7. And the absorption coefcient in the visible and UV light region is large, especially for ultraviolet light. In a word, this GaN/Janus-WSSe vdW heterostructure under a − 2% biaxial external strain is a potential solar photocatalyst for water splitting at pH=7.