A Pt-modified Au catalyst featured with novel layered structures and ultra-low Pt loading has been designed and electrochemically fabricated on a glassy carbon (GC) electrode. SEM characterization suggests that as-formed Pt/Au/GC electrode grows in a Stranski–Krastanov mode, resulting in a nearly ideal layered structure with Au at the inner layer and Pt at the outer layer. The electrocatalytic activity of the synthesized Pt/Au/GC electrode towards formic acid electrooxidation was studied, and comparative experiments with other modified electrodes (i.e., Pt/GC, Pt/Au, and Pt/Pt) were also conducted. As a result, the electrocatalytic activity of the outer-layered Pt depends significantly on the intrinsic properties of the substrates. The prepared Pt/Au/GC electrode with Au nanoparticles modified GC as the substrate shows remarkable catalytic activity for the formic acid oxidation, much higher than that of its counterparts, Pt/GC, Pt/Au, and Pt/Pt electrodes. Additionally, the measured electrochemical impedance spectra indicate that the charge-transfer resistance for formic acid electrooxidation on Pt/Au/GC electrode is smaller than that on other Pt modified electrodes.