The invention belongs to the field of medical chemistry, and particularly relates to a preparation method of impurity component cyanurodiamide pentose of azacitidine. The method is characterized in that N2, N4-bi(trimethylsilyl)-6-trimethylsilyl-1,3,5,- triazine-2,4-diamine which is adopted as a raw material has condensation reaction with tetra-o-acetyl-d-ribose under the protection of trimethylsilyl and under the catalysis of aluminium trichloride, boron trifluoride, zinc chloride or titanium tetrachloride at the temperature of -10 to 20 DEG C, and then a protecting group on the sugar is removed to prepare the cyanurodiamide pentose.