The large-scale synthesis of ternary nanocomposite hemin-graphene sheets/poly(3,4-ethylenedioxythiophene) (H-GNs/PEDOT) was achieved via a microwave-assisted method, in which PEDOT was polymerized with graphene oxide and induced with hemin on the H-GNs nanosheets without additional oxidants; meanwhile, graphene oxide was partially reduced simultaneously. The morphology and structure of the as-prepared nanocomposites were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, and ultraviolet-visible absorption spectroscopy. The H-GNs/PEDOT nanocomposite combines the main excellent properties of the three components, maintaining the peroxidase-like activity of hemin, and displaying a high-speed electron transfer ascribed to the presence of PEDOT and the graphene sheets. The H-GNs/PEDOT exhibited remarkable electrocatalysis towards the reduction of H2O2 due to the direct electron-transfer between the hemin and the modified electrode. The excellent synergic effect enables an enhanced electrochemical performance of the H-GNs/PEDOT modified electrode with good biocompatibility and fast redox property. The novel biomimetic H2O2 biosensor based on the well-designed ternary hybrid H-GNs/PEDOT has a low detection limit (0.08 μM) with a high dynamic response range (10-7 to 10-5 M) and a high sensitivity (235 μA mM-1 cm-2). The sensor displays good selectivity, repeatability, reproducibility and stability.