The invention discloses a novel preparation method of brexpiprazole, comprising: subjecting 7-hydroxy-1H-quinoline-2-one (2) as starting material and 1-chloro-4-bromobutane to substitution reaction to obtain 7-(4-chlorobutoxy)-1H-quinoline-2-one (3); subjecting the compound (3) and N-Boc piperazine to substitution reaction under the action of potassium hydroxide, and carrying out deprotection to obtain 7-(1-piperazine)-butoxy-1H-quinoline-2-one dihydrochloride (4); subjecting the compound (4) to substitution reaction under the action of isopropylmagnesium chloride to finally obtain brexpiprazole. The preparation method has the advantages of high product conversion rate and good product purity, is simple and easy to perform, high in yield and good in quality and facilitates industrial production.