The　rapid　development　of　electronic　devices　in　various　fields　has　stimulated　an　increased　demand　for　electromagnetic　interference　(EMI)　shielding　materials　working　in　ultra-wideband　frequencies　of　gigahertz.　However,　the　aggregation　problem　and　single　attenuation　mechanism　severely　limit　the　broadband　shielding　applications　of　nanomaterials.　To　address　these　challenges,　a　novel　composite　of　parallel　expanded　graphite　(EG)　embedded　in　the　densified　Fe3O4　matrix　is　designed　and　successfully　synthesized　through　the　in　situ　chemical　coprecipitation　followed　by　pressure　sintering.　This　unique　structure　creates　heterogeneous　interfaces　by　combining　the　conductive　EG　and　magnetic　Fe3O4.　The　significant　dielectric　and　magnetic　loss　then　arise　from　conduction　and　polarization　loss,　as　well　as　the　eddy　current　and　resonance　effects.　The　multiple　reflections　of　electromagnetic　waves　between　the　oriented　EG　sheets　further　cause　the　＂absorb-reflect-reabsorb＂　effect　that　prolongs　the　propagation　paths　and　enhances　the　dissipation　of　electromagnetic　waves.　Based　on　these　multiple　mechanisms　over　an　ultrawide　range　of　frequencies　(8.2-40　GHz),　the　EG/Fe3O4　composites　exhibit　exceptional　EMI　shielding　performance,　with　an　average　shielding　effectiveness　of　29.3　dB,　indicating　the　potential　for　multifunctional　EMI　shielding　applications.