Researchers have drawn attention to monolayer g-ZnO because of its broad absorption spectrum, but carrier recombination is an unavoidable problem for monolayer g-ZnO as a photocatalyst. How to reduce electron hole pair recombination rate and improve visible light utilization by monolayer g-ZnO are worth investigating, and builting heterojunctions and biaxial strain on them is a viable approach. As a result, this paper uses the first-principles method to investigate how biaxial strain affects the electronic structure and optical properties of g-ZnO/WS2 heterojunctions. The results show that the band gap of the g-ZnO/WS2 heterojunction is 1.646 eV, which reduces the recombination rate of photogenerated carriers due to the built-in electric field generated inside the heterojunction system. The edge of the heterojunction optical absorption spectrum, on the other hand, expands to the visible region. With the exception of the compressive strain (-2.5%) system, the absorption band edges of all strained systems show red-shift. With strain applied to the heterojunction increases, the degree of red-shift and the ability to bind charge of it increases. With the stronger strain on the heterojunction, the capacity of the hindrance of photogenerated electron-carriers recombination is stronger than that of the unstrained system. Moreover, its photocatalytic capability is also better than that of the unstrained system. The results show that the builting g-ZnO/WS2 heterojunction and biaxial strain on them have significant modulating effects on the electronic structure and optical properties of the heterojunction, making it useful for applications such as narrow-band, infrared and visible semiconductor devices, photocatalytic materials, and so on.