NOVELTY - Synthesis of cyclin-dependent kinase-4/6 inhibitor is Palbociclib involves reacting 2,4-dichloro-5-acetyl-pyrimidine and alkali, adding cyclopentylamine, separating organic phase, crystallizing, and filtering; melting fructone in tetrahydrofuran, cooling, adding lithium hexamethyldisilazide, potassium hexamethyldisilazide, or sodium hexamethyldisilazide, reacting with product in first step, extracting, combining organic phase, and drying; reacting with base and 5-tert-butyl-4-(6-amino-pyridin-3-yl)piperazine-1-carboxylate, separating solid, and filtering; and adding acid solution, reacting, and filtering.    USE - Method for synthesizing CDK-4/6 inhibitor is Palbociclib (claimed).    ADVANTAGE - The method has low cost and is simple and environment-friendly.    DETAILED DESCRIPTION - Synthesis of cyclin-dependent kinase (CDK)-4/6 inhibitor is Palbociclib comprises:    (A) reacting 2,4-dichloro-5-acetyl-pyrimidine and organic alkali such as triethylamine or diisopropylethylamine and inorganic alkali such as potassium carbonate, sodium carbonate, or sodium bicarbonate in solvent such as ethyl acetate, tetrahydrofuran, or methylene chloride, controlling temperature at - 5-80 degrees C, preferably - 5-5 degrees C, slowly adding cyclopentylamine at room temperature, stirring, water quenching, separating organic phase, washing with weak acid to adjust to pH 7, concentrating, crystallizing, and vacuum filtering;    (B) taking fructone, melting into anhydrous tetrahydrofuran, cooling to 0 degrees C to - 70 degrees C, preferably 0 degrees C to - 5 degrees C, slowly adding lithium hexamethyldisilazide (LiHMDS), potassium hexamethyldisilazide (KHMDS), or sodium hexamethyldisilazide (NaHMDS), stirring for 2 hours, adding product in first step, reacting at below 0 degrees C for 1 hour, naturally heating to room temperature for overnight, monitoring reaction by thin layer chromatography, quenching using saturated ammonium chloride, extracting with methyl t-butyl ether for 3 times, combining organic phase, washing with water once, concentrating, and drying;    (C) reacting with organic base such as triethylamine or diisopropylethylamine, inorganic base such as potassium carbonate, sodium carbonate, or sodium bicarbonate, and 5-tert-butyl-4-(6-amino-pyridin-3-yl)piperazine-1-carboxylate in dimethylformamide, toluene, or dimethyl sulfoxide, heating to 70-150 degrees C, reacting, cooling to room temperature, water quenching, separating light yellow solid, filtering, and washing filter cake with organic solvent; and    (D) adding to acid solution, stirring at room temperature, heating, reacting, and filtering.