The invention discloses a synthesis method of N-acetyl-L-cysteine, which belongs to the field of amino acid manufacture, and comprises the following steps of: dissociating commercially available industrial L-cysteine hydrochloride monohydrate serving as a starting material by using sodium hydroxide to obtain L-cysteine sodium salt, acylating with acetic anhydride to obtain N-acetyl-L-cysteine sodium salt, and finally, adjusting the acid with hydrochloric acid to obtain an N-acetyl-L-cysteine crude product, and refining the crude product with pure water to obtain an N-acetyl-L-cysteine finished product. According to the synthesis method disclosed by the invention, the material ratio and reaction conditions are optimized, on the basis of reducing the cost of the raw materials, the conversion rate is increased to 97% or above, the generation of related impurities is greatly reduced, the post-treatment operation steps are optimized, the efficiency is improved, and the reaction molar yield is increased to 80% or above under the condition that the product quality is not influenced.