Recently, gold nanomaterials and iron oxide nanoparticles (NPs) have attracted much attention due to their unique physical, chemical and biological properties. In this study, polyethyleneimine (PEI)-modified Fe3O4 NPs (inner cores) were first prepared, and then gold shells (AuNSs) were innovatively formed on the cores using epigallocatechin gallate (EGCG) as the reducing agent to obtain Fe3O4-PEI@AuNSs-EGCG core-shell NPs. After PEG modification, the Fe3O4-PEI@AuNSs-EGCG/PEG nanoplatform was completed. This highly integrated nanoplatform has shown remarkable biostability, rapid cellular uptake, and excellent in vitro and in vivo anti-tumour activity. Moreover, this nanoplatform can also be used as a magnetic resonance (MR)/X-ray dual-mode imaging agent. In all, the results of this study showed that Fe3O4-PEI@AuNSs-EGCG/PEG is a promising, potentially effective, theranostic nanoplatform for tumour treatment.