The invention discloses a preparation method of methyl heptenone. Under an alkaline condition, benzyl quaternary ammonium salt is used as a catalyst, prenyl chloride and acetone are synthetized into methyl heptenone through condensation reaction, salts in the reaction liquid are removed by filtering, by liquid separation, methyl heptenone in oil phase is obtained from upper liquid, and alkaline liquor in which the catalyst is dissolved is obtained from lower liquid. The preparation method has obvious positive effects, the benzyl triethyl ammonium chloride catalyst has relatively high selectivity to condensation reaction of prenyl chloride and acetone, and yield of methyl heptenone is more than 80%. The problem of failure in recycle of a catalyst in the prior art is solved radically. Since the benzyl quaternary ammonium salt has good dissolvability in the alkaline liquor, the catalyst is dissolved in the alkaline liquor after reaction, and in the alkaline liquor recycle process, the catalyst is recycled with the alkaline liquor so as to be reused.