NOVELTY - Synthesizing (S)-alpha-hydroxy-gamma-N-phthaloyl amino acid (I) involves: adding L-glutamic acid in acidic nitrite solution and carried out diazotization reaction to obtain (S)-butyric acid lactone (L1); and then carried hydrolysis, ammonolysis and Hoffman degradation reaction to form (S)-4-amino-2-hydroxybutyrate sodium solution (S1); adding phthalic anhydride into (S1), and then carried out ammonia hydrolysis reaction to form (S)-2-(3-carboxy-3-hydroxypropyl)carbamoyl)benzoic acid disodium; and then carried out cyclization reaction to obtain (I).    USE - For synthesis of (S)-alpha-hydroxy-gamma-N-phthaloyl amino acid (claimed) which is used as intermediate for the synthesis of amikacin drug.    ADVANTAGE - The present method solves the problem of existing method that requires multiple uses of the ion exchange resin for separation and purification, complex operation and high energy consumption. The present process provides final product having more than 60% of total yield, and more than 99% optical purity; provides product having stable quality; has mild reaction condition; doesn't provide environmental pollution; has low production cost; and is suitable for commercial preparation of (S)-alpha-hydroxy-gamma-N-phthaloyl amino acid.    DETAILED DESCRIPTION - Synthesis of (S)-alpha-hydroxy-gamma-N-phthaloyl amino acid involves: a) adding L-glutamic acid in acidic nitrite solution and carried out diazotization reaction to obtain (S)-butyric acid lactone; b) adding ammonia solution into (S)-butyric acid lactone and carried out alkali substitution reaction to form (S)-alpha-hydroxyglutaric acid mono sodium salt; c) subjecting the salt obtained in step b) to Hoffman degradation reaction to obtain (S)-4-amino-2-hydroxybutyrate sodium solution; d) adjusting the pH value of obtained solution, adding phthalic anhydride, and then carried out ammonia hydrolysis reaction to obtain (S)-2-(3-carboxy-3-hydroxypropyl) carbamoyl)benzoic acid disodium; e) and then carried out cyclization reaction to obtain (S)-alpha-hydroxy-gamma-N-phthaloyl amino acid.