Graphene has been widely used in many applications, such as sensors, field-effect transistors, and integrated circuits due to its high strength and excellent electrical and thermal conductivity. Its lack of a band gap, however, means that applications in some fields are limited. In this paper, using first principles calculations based on the density functional theory method, the electronic properties of graphene/WS2 heterostructures under electric field and in-plane biaxial strain are investigated. It is found that the two materials are subject to weak van der Waals forces after stacking. The band gap of WS2 in the graphene/WS2 heterostructure is reduced by 0.57 eV compared to the intrinsic WS2 band gap. An n-type Schottky contact with a barrier height of 0.22 eV is formed. In addition, an n-type to p-type Schottky contact transition can be achieved by increasing the applied vertical electric field on the graphene/WS2 heterostructure. The variation of the barrier heights ϕBn and ϕBp of the graphene/WS2 heterojunction with strain is sensitive. However, plane strain can only change the height of the Schottky barrier; the Schottky contact cannot change from n-type to p-type with in-plane biaxial strain. The results suggest that there is a potential application of graphene/WS2 heterostructures in Schottky devices.