The water-gas-shift reaction (CO + H2O → CO2 + H2) plays a key role in hydrogen economy. First-principles density functional theory has been used to investigate the mechanism of the water-gas-shift reaction on a model consisting of 3Cu atom cluster on an 3Cu/α-Al2O3(0001) surface. Three reaction mechanisms—redox, carboxyl, and formate—have been examined. After zero-point energy correction, our calculations show that the redox mechanism is controlled by l-CO2(a) formation and OH(a) diffusion. The carboxyl mechanism is dominated by the carboxyl formation. The OH(a) is a reactive intermediate and plays an autocatalytic role in catalytic WGS reaction. Specially, the OH(a) formation barrier can be reduced to 0.22 eV from the water dimer dissociation, and the H2(a) formation barrier is extremely low, 0.65 eV, on the 3Cu/α-Al2O3(0001) surface.