Abstract: Graphene film has been widely studied in the field of electromagnetic interference (EMI) shielding due to its unique layered structure, high conductivity, and good flexibility. However, graphene film has a large skin depth, which makes it difficult to balance high shielding effectiveness (SE) and thin thickness. To solve the above problems, the surface modification of graphene film and the design of multi-layer impedance mismatch interface were carried out based on the transmission line theory. The model of metal-graphene multilayer heterostructure was established for electromagnetic simulation. The influence of structural parameters on the EMI shielding performance was studied. According to the simulation model, Ag-graphene-Ag sandwich composite films (A-G-A CF) were fabricated by magnetron sputtering, and their EMI shielding performance and mechanism were investigated. After depositing Ag on graphene film, the conductivity increases by nearly two orders of magnitude, making the skin depth significantly decrease and the SE in 4~18 GHz increase from 24 dB to 44 dB. Moreover, the sandwich structure enhances the electromagnetic loss of the film and further improves the SE to 51 dB. With the increase of metal coating thickness, the SE of A-G-A CF gradually increases. The SE of A-G-A CF with a metal coating thickness of about 580 nm is as high as 65 dB, which can shield more than 99.9999% of electromagnetic wave energy.