Methanol dehydrogenation (MD) is highly valuable in hydrogen energy production, and the introduction of nonmetals has received much attention to improve the activity and stability of the MD catalysts, but the understanding of the role of non-metallic elements in catalyzing the MD reaction is rather limited. Density functional theory (DFT) is employed to investigate the mechanism of methanol dehydrogenation on RuxPy surfaces. In this work, the P element is introduced into the Ru-based catalyst to obtain dispersed Ru sites and RuxPy (x/y = 2 : 1, 1 : 1, and 1 : 2) catalysts are designed. CH3OH adsorption, electronic structure of the catalyst, energy barriers for carbon accumulation reactions, and the mechanism of methanol decomposition are systematically calculated. The results of the effective reaction barrier (Eeffa) reveal that the order of the activity of the MD reaction is RuP(112) > Ru(0001) > Ru2P(210) > RuP2(110). The most preferable pathway on RuP(112) is pathway 1 (CH3OH* → CH3O* → CH2O* → CHO* → CO*). After the introduction of P, the weakened CO adsorption enhanced the resistance of catalysts to CO poisoning, and the activation energy of the carbon accumulation reaction increased, indicating that the anti-coking ability of the catalysts is improved. This theoretical study contributes to the design and modulation of highly active and stable metal catalysts for MD reactions.