The present invention relates to a polyimide high dielectric composite material, wherein polyimide is adopted as a matrix phase, modified graphene and nanometer barium titanate are adopted as inorganic phases, components comprise, by mass, 0.5-3.0 w% of modified graphene, 9.6-29.8 w% of nanometer barium titanate, and 69.7-87.4 w% of polyimide, the modified graphene is p-phenylenediamine graft modified graphene, and the surface of the modified graphene contains chemically grafted polyimide. The preparation method comprises the following steps: (1) graphene oxide modification, (2) polyamic acid prepolymer preparation, and (3) composite material thermal cross-linking. According to the preparation method, the functionalized graphene and the polyimide molecules can be combined through a covalent bond form, electric conductivity of the graphene can directly affect dielectric property of the composite material, and a volume fraction of the barium titanate can be effectively reduced. In addition, the conventional solution blending technology is adopted, the preparation process is simple, operation is easy, and the method can be widely promoted and applied.