The invention discloses a preparation process for a key intermediate 1-(4-methoxyl)benzyl-1,2,3,4,5,6,7,8-octahydro
isoquinoline (mixed isomer) of
dextromethorphan hydrobromide serving as a cough relieving
medicine, which comprises the following steps of: performing
acylation reaction to obtain 1-(4-methoxyl)benzyl-2-alkylacyl-1,2,3,4,5,6,7,8-octahydro
isoquinoline (laevo isomer); and racemizing under the alkaline condition to obtain 1-(4-methoxyl)benzyl-2-alkylacyl-1,2,3,4,5,6,7,8-octahydro
isoquinoline (mixed isomer); and hydrolyzing under the alkaline condition to obtain the 1-(4-methoxyl)benzyl-1,2,3,4,5,6,7,8-octahydro isoquinoline (mixed isomer). The process aims to overcome the defect of the synthetic process and reduce cost, so that the process is simply and safely operated, the industrial production is qualified, and the yield of products is between 55 and 69 percent.