Crystalline defects in MoS2 may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single-layer graphene is sensitive to defect-induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal-boron nitride/graphene/MoS2 sandwich structure facilitates the exploration of the properties of those midgap states in MoS2. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS2/graphene interface. They are responsible for the contact problems of metal/MoS­2 interfaces.