Single atom alloy (SAA) catalyst usually shows high selective hydrogenation properties as compared to that of traditional metal catalyst, but the mechanism is still needed to be understood at the molecular level. The first-principles density functional theory (DFT) calculations were performed to study the selective hydrogenation of 1,3-butadiene into 1-butene catalyzed by sing Pt catalyst [Pt-1/Cu(1 1 1)] as well as high Pt content catalyst [Pt4-line/Cu(1 1 1), i.e. four Pt atoms in a linear type]. The calculation results indicated that the activity of H-2 dissociation as well as the H diffusion is facile on both Pt-1/Cu(1 1 1) and Pt4-line/Cu(1 1 1) catalysts, which means that the production of H species is not the key factor controlling the selectivity of 1,3-butadiene hydrogenation towards the 1-butene formation. The reaction path calculation results indicated that the kinetic behavior of hydrogenation of 1,3-butadiene to 1-butene is similar for both Pt-1/Cu(1 1 1) and Pt4-line/Cu(1 1 1) models, whereas the C-H bond scission of 1,3-butadiene displayed a significant difference on these two models, namely it is facile to broke C-H bond on Pt4-line/Cu(1 1 1) model, and thus leading to the lower selectivity for the 1-butene formation over Pt4-line/Cu(1 1 1). The reason behind the high activity of C-H bond scission on Pt4-line/Cu(1 1 1) comes from the fact that a small deformation energy of 1,3-butadiene molecule occurs on Pt4-line/Cu(1 1 1) surface during the process of C-H bond cleavage due to the relatively short C-H bond length at transition states as compared with that on Pt4-line/Cu(1 1 1) , and thus resulting in a relatively low C-H bond cleavage barrier on Pt4-line/Cu(1 1 1) surface. The present study may help people to design an efficient catalyst for selective hydrogenation of 1,3-butadiene by controlling the activity of C-H and/or C-C bond cleavage.