The invention discloses a synthesis method of Alectinib. The method comprises the following steps: carrying out a borating reaction between 6-bromo-3,4-dihydro-2-naphthalenone and n-butyl lithium and then an organic boron reagent; carrying out a catalytic coupling reaction between the obtained 3,4,-dihydro-2-naphthalenone-6-boric acid and bromoethane; carrying out a dimethylation reaction between the obtained 6-ethyl-3,4-dihydro-2-naphthalenone and iodomethane; carrying out a bromination reaction between the obtained 1,1-dimethyl-6-ethyl-3,4-dihydro-2-naphthalenone and a bromination reagent; carrying out a substitution reaction between the obtained 1,1-dimethyl-6-ethyl-7-bromo-3,4-dihydro-2-naphthalenone and 4-(4-piperidyl)morpholine; carrying out a cyclization reaction between the obtained 1,1-dimethyl-6-ethyl-7-[4-(morpholine-4-yl)piperidine-1-yl]-3,4-dihydro-2-naphthalenone and 3-cyanophenylhydrazine; and carrying out an oxidation reaction between the obtained 9-ethyl-6,6-dimethyl-8-[4-(morpholine-4-yl)piperidine-1-yl]-6,11-dihydro-5H-benzo[b]carbazole-3-formonitrile and dichlorodicyanobenzoquinone to obtain a finished product of Alectinib. The synthesis method has the advantages of relatively short route, simplified operation and relatively low cost and is a green and environment-friendly method suitable for industrial production.