• Methanol direct decomposition via CHOH* is the main reaction pathway at low temperature. • The surface is covered by CO at low reaction temperature. • High reaction temperature, low feed ratio of CH 3 OH/H 2 O and low reaction pressure are in favor of CO coverage decreasing. • The difference between water scission and methanol decomposition is the key factor for MSR reaction on Pt(111). Pt-based catalysts are widely used for hydrogen production from methanol steam reforming (MSR), but they show high CO selectivity. The reaction pathways of MSR on Pt(111) surface are systemically studied using density functional theory and kinetic Monte Carlo to reduce the selectivity of CO. The results show that CH 3 OH (g) → CH 3 OH* → CH 2 OH *→ CHOH * → CHO * → CO * → CO (g) is the main reaction pathway at 423–623 K and 0.1–10 atm with CH 3 OH: H 2 O of 3:1–1:1, and water is very difficult to dissociate. The difference between H 2 O* + * → OH* + H * and CH 3 OH* +*→ CH 2 OH * + H * is the key factor for MSR reaction on Pt(111) surface. The actual ratio of H 2 /(CO+CO 2) is close to the theoretical ratio for MSR (2.91 vs. 3.00) when the energy barrier of H 2 O* + * → OH* + H * is smaller than that of CH 3 OH* +*→ CH 2 OH * + H * by 0.30 eV. The elimination of CO* is mainly from CO * + OH * → COOH*, and the ratio of CO/(CO+CO 2) is 6.71%. These results may be useful for the design and optimization of Pt-based catalysts at low temperature for MSR reaction. [Display omitted] [ABSTRACT FROM AUTHOR]