The invention relates to the chemical synthesis of pharmaceutical API, and specifically to a method of synthesizing diclofenac sodium, which is a kind of nonsteroidal anti-inflammatory drug for relieving pain. The method includes: nitrating phenylacetate to prepare o-nitrophenylacetate (2); hydrogenating o-nitrophenylacetate (2) to prepare o-aminophenylacetate (3); amidating an amino group of o-aminophenylacetate (3) to obtain 2-(2-benzoylaminophenyl) acetate (4); 2-(2-benzoylaminophenyl) acetate (4) reacting with thionyl chloride to prepare a chloroimine intermediate, and then condensing the intermediate of chloroimine with 2,6-dichlorophenol using an inorganic base to prepare (E)-methyl-2-(2-((2,6-dichlorophenoxy)(phenyl)methyleneamino) phenyl ester (5); subjecting (E)-methyl-2-(2-((2,6-dichlorophenoxy)(phenyl)methyleneamino) phenyl ester (5) to Chapman rearrangement to afford methyl 2-(2-(N-(2,6-dichlorophenyl)benzoylamino)phenyl) ester (6); and hydrolyzing methyl 2-(2-(N-(2,6-dichlorophenyl)benzoylamino)phenyl) ester (6) to provide the target compound as of diclofenac sodium API. The overall yield is up to 67% based on methyl phenylacetate.