This invention reveals the method for preparing Palbociclib (I). The preparation includes: produce the 6-acetyl-5-methyl-2-substituent-pyrido[2,3-d]pyrimidin-7(8H)-one (IV) through the ring-closure reaction by using accessible raw materials of 1-(4-amino-2-substituent-5-pyrimidinyl) ethanone (II) and acetylacetic ester (III); produce the 6-acetyl-8-cyclopentyl-5-methyl-2-substituent-pyrido[2,3-d]pyrimidin-7(8H)-one (VI) through the substitution reaction between the intermediate (IV) and the cyclopentane halide (V); prepare the Palbociclib (I) through the condensation and hydrolysis reactions between the intermediate (VI) and 4-(6-amino-3-pyridinyl)-1-piperazinecarboxylic acid 1,1-dimethylethyl ester (VII). This method for preparing Palbociclib (I) is characterized by easily available raw materials, concise process and economy and environmental protection, and it is suitable for industrialized production.