This work reports the detailed mechanism of methanol decomposition on Pt3Ni(111) based on self-consistent periodic density functional theory calculations. The geometries and energies of methanol and its intermediates are analyzed, and the decomposition network is mapped to illustrate the decomposition reaction mechanisms. On Pt3Ni-(111), the less electronegative Ni atoms are more favorable for adsorbing radical intermediates and intermediates with lone pair electrons (such as O-containing species). The possible pathways through initial scission of the O-H, C-H, and C-O bonds in methanol are studied and discussed based on the steric effect and electronic structure of the related transition states and the Bronsted-Evans-Polanyi (BEP) relationships. The initial scission of the O-H bond is the most favorable and bears the lowest energy barrier among the three decomposition modes (initial scission of O-H, C-H, and C-O bonds). The decomposition of the energy barrier analysis indicates that the high energy barrier for initial C-H and C-O bond scission is caused by the large structural deformation, strong repulsive interaction, and the low adsorption ability of the decomposed species in their transition states. Potential energy surface (PES) analysis confirmed that the favorable decomposition pathway for methanol on Pt3Ni(111) proceeds via CH3OH -> CH3O -> CH2O -> CHO -> CO, in which scission of the O-H bond is the rate-limiting step. The comparison between the current results and CH3OH decomposition on other systems shows that Pt3Ni(111) can efficiently promote methanol decomposition and alleviate the CO poisoning problem when it is used as an anode catalyst in direct methanol fuel cells (DMFCs).