The invention discloses a polymer which can perform addition reaction with thiol and has stable hydrolysis. The polymer is formed by introducing at least one functional group shown in a formula (I) to a chain side and/or a chain end of a hydrophilic polymer, wherein X is any group or polymer chain. The polymer has the advantages that: the polymer can react with proteins or polypeptides under physiological environment without other micromolecule components, so the probably brought biotoxicity is avoided and the proteins to be modified are not influenced; and ester bonds, amido bonds, amido ester bonds and other easily hydrolytic chemical bonds are absent between the chemical bond formed after the addition reaction and a main chain, so the hydrolysis is stable.