In this work, a novel 3D nanocomposite is presented, which is consisted of poly(N-acetylaniline) (PAANI)/multi-walled carbon nanotubes (MWNTs) nanorods (in which the inner layer was comprised of MWNTs and the outer layer was PAANI, forming a core-shell structure) and the decorated Pt nanoparticles. Electrochemical techniques (such as electrochemical impedance spectroscopy (EIS)), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to characterize the nanocomposite. The resulting Pt/PAANI/MWNTs nanocomposite can be served as a higher efficient catalyst for the electrochemical oxidation of formaldehyde at 0.41V with the catalytic current of 0.73mA, which was accompanied with about 45mV potential negative shifts and two-fold increase in peak current comparison to the Pt/PAANI. These results reveal that the Pt/PAANI/MWNTs nanocomposite film is quite promising in the fuel cell applications.