The addition of alkali metal promoter is significant to improve the performance of copper catalyst in water–gas shift reaction (WGSR), but the previous theoretical work mainly focuses on the single crystal metal model like Cu(1 1 1), rather than the real supported copper clusters. In this paper, Cu₄/TiO₂(sub-nanometer), Cu_n/TiO₂ (copper nanorod) were chosen to explore the effect of K on WGSR where K is located on TiO₂ support, Cu/TiO₂ interface, and copper clusters surface. Density functional theory calculation results show that only when K is on Cu_n/TiO₂ and Cu(1 1 1) surfaces, can K promote water–gas shift reaction. Microkinetic modeling analysis shows that K mainly promotes the water–gas shift reaction by promoting the decomposition of H₂O, and has little influence on the C-O bond formation like carboxyl and CO₂. Combined with energetic, electronic and geometric analysis, on the surfaces of Cu_n/TiO₂ and Cu(1 1 1), K mainly enhances the adsorption strength of H₂O and reduces the barrier of H₂O dissociation through direct bonding with oxygenate species, thereby improving the performance of WGSR. The copper clusters with sub-nanometer size cannot reveal the promotion effect of K, and the microkinetic modeling results of Cu(1 1 1) surface do not conform to the real situation, but the copper nanorod model can appropriately reflect the promotion effect of K. The structure unit of the active center is [TiO₂−Cu−K]. The promotion of other alkali metals was explored, and the microkinetic modeling results show that the order of alkali metal promotion is as follows: Li < Na < K < Rb < Cs. The electronic structure analysis shows that the smaller the electronegativity of alkali metal, the lower the surface work function, the higher the surface dipole moment, and the stronger the promoting effect of alkali metal. This work not only reveals the origin of the K effect on WGSR, but also provides the accurate placement of K promoter.