A model based on the perfectly stirred reactor (PSR) concept is used to correlate the minimum extinguishing concentration (MEC) of gaseous KOH, which was established in the cup-burner experiment. Both physical and chemical mechanisms of fire suppression are considered in PSR modeling. The combustion process of the CH4/air and gaseous KOH mixture contains a complex chain branch reaction. A high gaseous KOH concentration of type K agent in the methane flame results in strong flame inhibition. The influence of gaseous KOH on the combustion process involves capturing OH free radicals in the flames and reaching the inhibition effect by controlling key elementary reactions containing OH radicals in the combustion process. By calculation of the intermediate product production rate of the fire-extinguishing agent at a high temperature through the PSR model, the main paths of gaseous KOH inhibiting the methane combustion reaction can be obtained. An increased KOH concentration will not change the reaction mechanism.