Periodic DFT calculations have been performed to systematically investigate the mechanisms of methanol decomposition and oxidation on the PtRu/Pt(111) surface. Geometries and energies for the primary species involved are analyzed and the reaction network has been mapped out. The calculation shows that among three initial Csingle bondH, Osingle bondH, and Csingle bondO bond scissions of methanol, the Osingle bondH bond scission is found to be the most favorable and bears a lower energy barrier than the desorption of methanol. The decomposition of CH₃O occurs via the path CH₃O → CH₂O → CHO → CO with the limiting step of the first dehydrogenation. Although the oxidation of CO is hindered by a high barrier, the CHO oxidation to CHOOH could occur facilely. Further decomposition of formic acid to CO₂ and/or CO could occur via four possible pathways, that is, initial Csingle bondH, Osingle bondH, and Csingle bondO bond activations as well as simultaneous activation of Csingle bondH and Csingle bondO bonds, where the first pathway, HCOOH → COOH → CO₂, is the most favorable from a kinetic point of view. Compared to that on Pt(111), methanol on PtRu/Pt(111) prefers to decomposition rather than desorption and then oxidation via the favorable non-CO path with a lower rate-determining energy barrier of CH₃O → CH₂O for the whole reaction, which indicates that PtRu alloy can improved tolerance toward CO poisoning compared with pure Pt.