In view of the limitation of a single absorbing material to improve the microwave heating property of concrete, graphite powder and Fe3O4 were incorporated into concrete as composite absorbing material. The appropriate content of the composite absorbing material was determined by mechanical properties test. The effect of composite absorbing material on microwave heating property of concrete was studied by microwave heating test. The influence mechanism of the composite absorbing material was analyzed by electromagnetic parameter test. Meanwhile, the results were compared with those of the separate addition of graphite powder and Fe3O4. The results show that the addition of graphite powder and Fe3O4 can reduce the mechanical properties of concrete, and the content of absorbing material should not exceed 6% (mass fraction). The microwave heating property of concrete increases with the increase of separate addition of graphite powder and Fe3O4. When graphite powder and Fe3O4 are incorporated, the microwave heating property of concrete is better, and the best mix ratio is m(graphite powder)∶m(Fe3O4)=2∶1. When graphite powder and Fe3O4 are incorporated, the complex permittivity, dielectric loss angle tangent, complex permeability and magnetic loss angle tangent of concrete significantly increase, and the dielectric loss capacity and magnetic loss capacity significantly increase at the same time.