We studied the mechanism of the water−gas shift reaction (WGSR; CO + H 2 O → CO 2 + H 2 ) catalyzed by Co 6 @Au 32 core−shell nanoalloy using density-functional theory (DFT) calculations to investigate the bimetallic effects on the catalytic activation. The molecular structures and adsorbate/substrate interaction energies were predicted, along with the potential energy surface constructed using the nudged elastic band (NEB) method. Our results indicated that the energetic barriers of the two hydrogen dissociation reactions are lower on the core−shell nanoalloy than on Au 38 . Furthermore, all of the related chemical species of the WGSR can adsorb stably on Co 6 @Au 32 to allow the reactions to take place under ambient pressure. To gain insight into the synergistic effect in the catalytic activity of the Co 6 @Au 32 nanoalloy, the nature of the interaction between the adsorbate and substrate was analyzed by detailed electronic local densities of states (LDOS) as well as molecular structures.