The invention relates to the field of pharmaceutical synthesis process and discloses a method for synthesizing azacitidine. The method comprises the following steps: performing silanization reaction on bis(trimethyl disilylamine) and 5-azacytosine by taking toluene as solvent under the catalysis of phase transfer catalyst of quaternary ammonium compounds under a moisture-isolated condition to generate a compound with structure of formula II; performing condensation reaction between the compound and ribose tetracetate to generate a compound with a structure of formula IV; and performing alcoholysis on the compound in an alkaline environment to generate the azacitidine. The synthesis method of the invention shortens the time of silanization reaction, reduces the waste amount of the bis(trimethyl disilylamine) and avoids influence of the moisture in methanol and ethanol in the purification stage on the purity of the finished product of azacitidine; moreover, the synthesis method has the advantages of cheap reagents, few reaction steps and mild reaction conditions and is favorable for industrial production.