The {PEI-GNs/PMo₁₂}_n composite films are prepared by an LBL electrostatic assembly technique, and their uniform and homogeneous traits have been verified by UV-vis and cyclic voltammetry. The {PEI-GNs/PMo₁₂}_n composite films are used as a novel catalyst support for electro-deposition of Pt particles in situ. Pt particles immobilizing on the surface of {PEI-GNs/PMo₁₂}_n composite films were confirmed by XPS, EDX, XRD and FE-SEM. The interesting seashell-like Pt micro–nano-clusters can be obtained when {2PEI-GNs/PMo₁₂} composite films are used as catalyst supports. Pt-{PEI-GNs/PMo₁₂}_n composite films present good electrocatalytic activity regarding methanol oxidation and improved tolerance of CO. These results indicate that poly(ethyleneimine) functionalized graphene (PEI-GNs) preserves the superior electric conductivities of graphene sheets in the composite films, and the introduction of electroactive [PMo₁₂O₄₀]³⁻ (PMo₁₂) into the composite films helps to convert intermediate species CO into CO₂ from the Pt nanoparticles. The synergistic effect of PEI-GNs and PMo₁₂ remarkably enhanced the electrocatalytic activity and stability regarding methanol oxidation.