Direct ethanol fuel cells (DEFCs) promise the use of ethanol as a bio-renewable and non-toxic fuel for energy conversion through the ethanol oxidation reaction (EOR). Well dispersed Pt3Sn and Pt nanoparticles on graphene (denoted Pt3Sn/G and Pt/G) support electrocatalysts made with alcohol reduction were tested towards EOR. The HRTEM and XRD characterizations provide the morphology and crystal phase of the Pt3Sn alloy nanoparticles, which has a uniform particle size of 2.8±0.08 nm, as is consistent with a Pt/G catalyst as reference. According to the in-situ quick X-ray-absorption near-edge structure (QXANES) spectra during an anodic scan of CV for the EOR test to explain clearly the potential-dependent electronic state of the prepared electrocatalysts, the white-line intensities of the Pt L3-edge QXANES spectra and their spectral profiles vary appreciably with the electrode voltage. Moreover, Pt3Sn/G shows a better EOR performance than Pt/G because SnO2 can improve adsorption and dissociation during the oxidation by an appropriate expansion of the lattice parameters in the PtSn alloy. This work provides insight into the reaction mechanism of dissociative adsorption of ethanol on alloyed Pt surface, which has an important role in enhancing the EOR activity for a complete ethanol oxidation.