A synthetic process of mivacurium chloride (1) was optimized. 3, 4, 5-Trimethoxyphenylacetic acid (7) and 3, 4-dimethoxyphenylethylamine were used as starting materials to obtain the key intermediate (R)-5'-methoxylabdanin[(R) -2] dibenzoyl tartrate through dehydration condensation, intramolecular cyclization, imine reduction, reductive amination, and chiral resolution. Compound (R) -2 dibenzoyl tartrate reacted with 1, 3-propylene glycol cyclic sulfate under alkaline conditions via nucleophilic substitution and hydrolysis to obtain its quaternary ammonium alcohol (3). Finally, compound 3 was condensed with trans-4-octene-1, 8-dicarboxylic acid to obtain 1. In the post-treatment of the intramolecular cyclization step, omission of alkaline solution for quenching phosphorus oxychloride effectively prevented the formation of oxidative impurities. The synthesis of (R) -2 dibenzoyl tartrate was improved by reducing two resolutions to a single step, increasing the resolution yield from 33% to 42%. Replacing 3-chloropropanol with 1, 3-propylene glycol cyclic sulfate during the synthesis of compound 3 eliminated polymer impurity A and simplified the process by omitting column chromatography. The optimized process achieved a total yield of 17.0% (based on 7), and a purity of 99.6%. It was verified by kilogram-level amplification and was suitable for industrial production.