The invention discloses a synthetic method of PHBA, and belongs to the technical field of pharmaceutical synthesis. According to the method, L-glutamic acid is taken as the raw material, and is subjected to diazotization, hydrolysis, aminolysis and Hoffman degradation to obtain (S)-4-amino-2-hydroxybutyrate which is not purified and used directly for synthesizing (S)-alpha-hydroxyl-gamma-N-phthaloylaminobutyric acid. The synthetic method solves the problems that the conventional method is cumbersome in operation, high in energy consumption and serious in environmental pollution due to the fact that ion exchange resin is required to be used repeatedly for separation and purification of a product, steps of reactions are organically combined, the whole process is considered, and numerous distillation and purification processes are omitted, so that the whole process is greatly simplified, the total yield of the product is larger than 60%, the content of the product is larger than 99.0%, the optical purity is higher than 99.0%, the quality is stable, reaction conditions are mild, pollution of the three wastes is little, the preparation cost is low, and the method is suitable for industrial preparation of (S)-alpha-hydroxyl-gamma-N-phthaloylaminobutyric acid.