The invention provides a synthesis method of phosphorus pentachloride. In the method, with liquid phosphorus trichloride and chlorine as raw materials, a phosphorus pentachloride finished product which is uniform in particle size, good in fluidity, high in content and low in residual phosphorus trichloride content is obtained through the three stages that a reaction is carried out under stirring and normal-pressure chlorine introduction, the reaction continues under stirring and chlorine introduction pressure maintaining, and the reaction continues under stirring and negative pressure. Equipment used in the synthesis process is simple, and the obtained product is good in quality and has high industrial production value.