Abstract: The activation of hydroxyl group is a key step in the synthesis of ligand-polymer for affinity partition. Using thionyl chloride as the chloridating agent and polyethylene glycol (PEG) as the model polymer, the conventional process was improved by introducing triethylamine(Et₃N) into the reaction system.The precipitate of Et₃N•HCl formed in the system promotes the reaction into a thermodynamically beneficial process, which significantly reduced the dosage of thionyl chloride and simplified the synthesis procedure.The mole ratio of reaction materials（SOCl₂，Et₃N and PEG），reaction temperature and reaction time were investigated and optimized.A high value of the synthetic yield was reached at 1.54 mol Cl •(mol PEG)^-1.The comparisons to the conventional thionyl chloride method and epichlorohydrin method were also discussed.