Explosion prevention and mitigation technologies for hydrogen-methane gas mixtures represent a critical research area for ensuring the safe application of hydrogen energy. This study systematically investigates the inhibition mechanism of potassium bicarbonate (KHCO₃)-containing fine water mist on methane-hydrogen premixed deflagration using a combined approach of experiment and numerical simulation. The results indicate that KHCO₃-containing fine water mist exhibits a significant inhibitory effect on methane-hydrogen premixed deflagration, with its suppression performance positively correlated to the KHCO₃ mass fraction. Taking the condition of H₂ volume fraction of 10% as an example, 11% KHCO₃ addition resulted in reductions of the maximum explosion pressure and the average pressure rise rate by 34.64% and 44.57%, respectively. The laminar burning velocity was reduced by up to 66.43%. KHCO₃ contributes to suppression through both physical and chemical mechanisms. Physically, droplet phase change (evaporation) absorbs heat and the generated steam dilutes the fuel mixture, thereby lowering the flame temperature and reducing reactant concentrations. Chemically, the decomposition of KHCO₃ generates potassium compounds, which undergo the KOH→K→KOH recombination cycle to scavenge key radicals (·H, ·O, ·OH). This process competes with chain-branching reactions and interrupts the combustion chain reactions. Furthermore, the suppression process is governed by a competition between inhibitory and promotional effects. At high hydrogen blending ratios and high mass fractions of KHCO₃, the physical evaporation efficiency becomes a bottleneck that constrains the chemical inhibition, leading to a saturation of the overall suppression efficiency. Nevertheless, a significant inhibitory effect is still maintained.