A method for preparing a precursor used to label hepatocyte receptors is revealed. The precursor contains a bifunctional structure including trisaccharide and DTPA ligand. During synthesis processes of the precursor, silica gel columns and Reverse phase-18 (RP-18) columns are used for purification. Thus both the purification times and cost of each purification are reduced. Moreover, use diethyl ether to facilitate precipitation of products and remove a part of coupling reagent. Removing the coupling reagent helps purification of products. Furthermore, Nα,Nα-bis(carboxymethyl)-L-lysine hydrate and benzyl chloroformate are coupled to form a trisaccharide skeleton so as to ensure the yield rate of trisaccharide structure.