The invention relates to the field of medicament synthesis and discloses a synthetic method of decitabine. The method comprises the following specific steps of: undergoing a condensation reaction on 2,4-di-(trimethyl silicane)-5-aza-cytimidine and chloro-ribose under the catalytic action of nitrophenol; and hydrolyzing in an alkaline environment for removing acetyl to generate decitabine. In the synthetic method of decitabine, disclosed by the invention, nitrophenol is taken as a catalyst, the ratio of a beta configurational isomer in the obtained chloro-ribose aza-cytimidine is large, the content of beta configurational decitabine generated after hydrolysis is high, the purifying difficulty of decitabine is lowered, and the purity of a decitabine finished product is increased. During purification, ethyl acetate is used instead of methanol or ethanol, so that the influence of methanol or ethanol on the purity of the decitabine finished product at the purifying stage is avoided; and moreover, the synthetic method has the advantages of cheap reagents, less reaction steps, mild reaction condition and contribution to industrial production.