Metallic Sn has triggered significant research efforts as a lithium ion battery anode due to its high theoretical capacity and low-cost. However, the structural damage induced by the severe volume change of Sn and the continuous interfacial reaction due to the electrolyte remain two major challenges to hinder the practical applications of Sn-based anodes. In this work, we report the fabrication of a novel 3D composite architecture that is constructed by flexible graphene/Sn sandwich nanosheets synthesized by an in situ catalytic synthesis strategy, which relies on in situ formation and spatial entrapment of active Sn nanoparticles (NPs) inside the simultaneously catalytically formed 3D porous graphene. Such a unique architecture not only provides robust protection against the aggregation and volume changes of Sn NPs, and thus effectively avoids the direct contact between entrapped Sn and the electrolyte and enables the interfacial and structural stabilization of entrapped Sn NPs during cycling, but also ensures highly favorable three dimensional transport kinetics for both electrons and lithium ions in the whole electrode. As a result, this 3D composite anode exhibits very high reversible capacity, superior rate capability, and extremely excellent cycle stability even at high rates (650 mA h g⁻¹ at 2 A g⁻¹ over 500 cycles).