In the present work, density functional theory (DFT) calculations were performed to explore the reaction mechanism and activity of the water gas shift reaction (WGSR) on a clean and K-promoted Pt₄₀ nanorod supported by the ZrO₂ (Pt₄₀/ZrO₂) model, as well as on a clean and K-modified Pt(111) surface. The calculation results show that the carboxyl mechanism is responsible for the WGSR with H₂O dissociation as the common key step on all models. We found that the Pt₄₀/ZrO₂ model is more active toward the WGSR compared to the Pt(111) surface due to the central roles of the support and interface on Pt₄₀/ZrO₂ which can facilitate H₂O dissociation by strengthening OH binding at the transition state (TS) and final state (FS). More importantly, it is noticed that the addition of K can enhance the activity of the WGSR on the Pt₄₀/ZrO₂ model by reducing the apparent activation energy of the whole reaction as well as the energy barriers of the steps, i.e., H₂O and COOH dissociation for the dominant carboxyl pathway. The origin of the K promotion effect on H₂O and COOH dissociation is that K can greatly stabilize the dissociated oxygenated species at the TS by the direct K–O bonding. Nevertheless, the K adatom would hinder the progress of the WGSR on the Pt(111) surface with one key reaction of H₂O dissociation slightly promoted and another vital reaction of COOH formation strongly poisoned by K addition. The stronger promotion effect of K on H₂O dissociation on Pt₄₀/ZrO₂ than that on Pt(111) is attributed to the smaller stabilization effect of K on H₂O on Pt₄₀/ZrO₂, indicating that the K effect on the dissociation reaction is structure sensitive. Moreover, the K effect is also sensitive to the reaction type since K promotes nearly all the dissociation reactions but neutrally affects or even inhibits the kinetics of the association reactions on both Pt₄₀/ZrO₂ and Pt(111) models, caused by the same or even higher stabilization effects on the ISs from K compared to the corresponding TSs. The functional mechanism expounded in this work could be applicable to other electropositive additives like Na and Cs in the activation of the WGSR.