The invention provides a PMMA resin with high heat resistance and high impact resistance and a preparation method thereof, belonging to the technical field of modified resin. The resin comprises the following components by weight: 60-80 parts of PMMA, 0.1-2 parts of nanometer graphene, 20-40 parts of a toughening agent, 0.1-1 part of an antioxidant, 0.1-1 part of a lubricant and 0.1-1 part of a weatherproof agent. Compared with the prior art, the invention has the following advantages that since the nanometer graphene material is added into the PMMA resin and a silicon-containing acrylate-epoxy modified MMA polymer with a core-shell structure is added as the toughening agent, the prepared PMMA resin is prevented from deformation during high-temperature heat baking or heat storage on the premise of ensuring high impact resistance; since the size of the nanometer graphene is very small, the nanometer graphene does not affect the transparency of PMMA and has no influence on high gloss, blackness and brightness of black products; and the prepared PMMA resin is applicable to automotive exterior parts like grills, columns and rearview mirrors with high requirements on heat resistance, impact resistance and appearance.