This invention relates to a preparation method of propylene oxide. In existing technology takes low concentration H2O2 water solution as oxidizer, phosphorus tungsten heteropoly acid quaternary ammonium salt as catalyzer to prepare propylene oxide. This invention mainly solve the problem in existing technology of separation energy high caused by water, as well as catalyst stability low caused by decompound of phosphorus tungsten heteropoly acid quaternary ammonium salt, and problem of H2O2 activity factor descending. This invention takes propylene and aqueous hydrogen peroxide solution of weight percent concentration 40 to 80 as raw material, takes phosphorus tungsten heteropoly acid quaternary ammonium salt as catalyzer, takes at least one of toluene, benzene, ethylbenzene, cumene or chloroform as solvent, under 20 to 100deg reaction temperature and 0 to 3.0 Mpa reaction pressure, to carry out reaction for 0.5 to 24 hours. It best solve existing problem, can be used in industrial production of propylene oxide.