The invention discloses a synthesis method of synthesizing florfenicol. The synthesis method comprises the following steps: L-threo-(p-(methylsylfonyl) phenyl) serine ethyl ester which is used as raw material is processed by the protection by a protecting group, configurational transition, hydrolyzation, acetylation, re-protection, deoxidation, fluorination, and pre-hydrolyzation, so that the florfenicol is obtained, wherein, the protecting group R is one of benzoyl chloride, phthalic anhydride, cyanophenyl, and allyl compounds; the configurational transition is performed when sodium alcoholate or sodium methoxide exists. The raw material which is used in the method is a byproduct which is generated during the process of preparing thiamphenicol, is easy to get in the market, and is inexpensive; the technological operation is simple, the cost is low, the yield rate is high, and the synthesis method has industrialized value.