The invention relates to a preparation method of an intermediate needed for preparing a Bcl-2 inhibitor venetoclax. The synthetic route is represented as follows, and specifically comprises the following steps: using a compound I 2,4-difluorobenzoic acid as a raw material, performing hydroxylation and methyl esterification reactions to obtain a compound II methyl 4-fluoro-2-hydroxybenzoate; performing coupling on the compound II and piperazine to obtain a compound III; performing a reaction on the compound III with a raw material IV to obtain a compound V; and performing C-O coupling on the compound V and a compound VI 5-bromo-7-azaindole to obtain a compound VII, which is the intermediate. The preparation method provided by the invention has a mild reaction condition, has simple operation, is environmental friendly and high efficient, has low cost and is easy for large-scale production.