Graphene devices are susceptible to the surrounding environment^1-4. For example, the substrate in contact with graphene influences the device performance because the carriers are confined in two-dimensional (2D) atomic thickness. However, 2D van der Waals dielectric materials used as an interface modifier can provide a path to improve the device quality. In this paper, we report an enhanced mobility of 35000 cm² V^-1 s^-1 in a CrOCl-coated monolayer graphene on the SiO₂/Si substrate through a dielectric shielding effect. When monolayer graphene is sandwiched between two layers of 2D CrOCl insulator, the enhanced mobility can be 70000 cm² V^-1 s^-1. Because of the enhanced mobility, the Shubnikov-de Haas (SdH) quantum oscillation is also observed with the amplitude linearly decreasing with increasing temperature, consistent with the standard Lifshitz-Kosevich theory, and the effect persists at temperatures up to 100 K. Our work paves a way to improve graphene mobility and realize nontrivial quantum states at high temperature for the exploration of potential applications in electronics.