Using L-aminopropanol (2) as raw material, it was protected by o-nitrobenzenesulfonyl chloride, and substituted with 3-amino-1-propanol, the secondary ammonia was protected with tert-butoxyformyl (Boc). After intramolecular Mitsunobu cyclization reaction, o-nitrobenzenesulfonyl protector was removed with dodecanthiol to produce Ripasudil key intermediate (S)- 3-methyl-1, 4-diazo-1-tert-butylcarbonyl (1). The improved process is shorter and easier to operate, the total yield is increased from 19% to 47%, and the chiral isomer impurity content is less than 0.1%. This improved route has been used in pilot scale production.