The invention relates to a preparation method of a graphene-coated manganese dioxide composite wave-absorbing agent, belongs to the technical field of electromagnetic wave-absorbing materials, and solves the technical problems of weak binding force, poor composite interface performance and serious secondary agglomeration of heterogeneous materials, and the specific preparation process comprises the following steps: S1, weighing manganese sulfate and ammonium persulfate; s2, slowly adding the manganese sulfate into a graphene oxide aqueous solution at a constant speed, and carrying out magneticstirring at the same time, so as to prepare gel; s3, slowly adding the ammonium persulfate into the prepared gel at a constant speed, and magnetically stirring at the same time; s4, performing hydrothermal treatment on the mixed solution; and S5, separating a reaction product, washing the reaction product with deionized water until the reaction product is neutral, and freeze-drying the reaction product to obtain the graphene-coated manganese dioxide composite wave absorbing agent. The two components are tightly compounded in a chemical bond mode, the dielectric loss performance of the manganese dioxide to incident electromagnetic waves is brought into play, the multiple scattering performance of reduced graphene to the incident electromagnetic waves is combined, the preparation process issimple, and industrialization is easy to achieve.