SnS2 with atomic thickness has attracted extensive attention in the field of photocatalysis due to its special physicochemical properties, suitable band gap, low cost and low environmental toxicity. However, the application of SnS2 in the field of optoelectronics is restricted by its low photocatalytic efficiency and carrier mobility. In this study, vacancies and transition metal atoms are introduced into a SnS2 monolayer to modulate its physicochemical properties. Meanwhile, the SnS2 monolayer modified by vacancies and transition metal atoms is combined with graphene to form a heterostructure, which promotes the separation of photogenerated electron-hole pairs. The results of theoretical calculations show that the SnS2/graphene heterojunction can promote the separation of photogenerated carriers in intrinsic monolayer SnS2, and improve the photocatalytic efficiency and carrier mobility. The modification of Sn vacancies and Fe, Co atoms not only expands the visible light response range of the SnS2/graphene heterojunction, but also introduces magnetism, which is expected to be applied in spin optoelectronic materials. In this work, defects, doping and heterojunction assembly are rationally integrated, which provides a new idea for the design and development of spin optoelectronic devices based on monolayer SnS2.