Provided is a method of purifying a polyarylene sulfide by washing it with a mixture of an aprotic organic solvent and a salt prepared through reaction of a strong acid with a weak base, followed by further washing the polyarylene sulfide with a mixture prepared by adding a halogenoaromatic compound having an electron-withdrawing group to an aprotic organic solvent of a temperature of from 150°C to 300°C. The method makes it possible to reduce the impurities such as alkali metal halides in polyarylene sulfide, thus giving thermally stable polyarylene sulfide.