The invention discloses a preparation method of potassium peroxymonosulfate. The preparation method comprises following steps: adding a potassium peroxydisulfate solution into a hydrolysis reactor, carrying out hydrolysis under the action of a catalyst to generate potassium peroxymonosulfate and potassium hydrogen sulfate; after the hydrolysis degree reaches the requirement, removing the catalystfrom the solution by a filter, adding the solution into a neutralization reactor, adding a stabilizing agent into the neutralization reactor, then adding potassium hydrogen sulfate solids, stirring, mixing; then adding potassium hydroxide into the neutralization reactor to carry out neutralization reactions; carrying out vacuum concentration, cooling, crystallizing, performing centrifugal separation, and baking to obtain a dry product of potassium peroxymonosulfate. Potassium peroxydisulfate is used to replace high concentration hydrogen peroxide, reacts with fuming sulfuric acid, and is hydrolyzed to generate potassium peroxymonosulfate and potassium hydrogen sulfate under the action of a catalyst; the reactions are mild, the reaction heat is little, the danger of reactions is reduced effectively; under the action of the catalyst, the degree of directional hydrolysis is high, the active oxygen content of products is qualified, and the product quality is stable.