Monolayer molybdenum disulfide (MoS2), a semiconductor material with direct band gap, is considered to be an important fundamental material for the future development of the semiconductor industry. In order to apply the material to semiconductor devices, we have to investigate the electrical, optical and thermal properties of MoS2. People have always been concerning about the electrical and optical properties, but pay little attention to the thermal properties of MoS2, especially thermal stability. It is well known that semiconductor device generates a lot of heat when it works, sometimes even running in high temperature environment. The above conditions all require the material which has good thermal stability. So we focus on how to improve the thermal stability of MoS2. In this paper, we report the construction of the van der Waals heterostructures of graphene and MoS2 by encapsulating monolayer MoS2 with graphene, and dissect the thermal stability of encapsulated MoS2 in argon (Ar) and hydrogen (H2) atmosphere respectively. The results show that in Ar atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, while the exposed MoS2 is decomposed almost completely at 1000 ℃. In H2 atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, but the exposed MoS2 is decomposed completely at 800 ℃. In conclusion, the thermal stability of MoS2 encapsulated by graphene can be improved significantly. We analyze the reason why MoS2 encapsulated by graphene gains good thermal stability. Firstly, the covered graphene provides additional van der Waals forces, which increases the decomposition energy of MoS2, making it more stable at high temperature environment. Secondly, graphene separates MoS2 from the external environment, preventing MoS2 from contacting and reacting with external gas, which greatly improves the thermal stability of MoS2 at high temperature environment. Meanwhile, graphene covers the active defect site on MoS2, making it difficult to react at defects. In summary, the monolayer MoS2 devices can work normally at high temperature when MoS2 is encapsulated by graphene. In addition, our work also provides a feasible approach to improving the thermal stability of other two-dimensional materials.