Orthogonally protected disaccharide building blocks for synthesis of heparin saccharide are disclosed. The disaccharide building block has a formula (I), in which L is a leaving group, P1, P2, P3 and P4 are different, and of them P1 is an ester-type protecting group, P2 is a hydroxyl protecting group that could be oxidized to a carboxylic acid, P3 is a hydroxyl protecting group, and P4 is a hydroxyl protecting group which allows chemoselective deprotection with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Acting as an elongation unit, the disaccharide building block of formula (I) may react with a starting unit of formula (II) to synthesize a heparin saccharide of desired size.