A phenolic syntan GA-SHBS had been synthesized by radical copolymerization of gallic acid (GA) and sodium 4-hydroxybenzenesulfonate (SHBS) using horseradish peroxides (HRP)/H2O2 as catalyst. The effects of monomer ratio, reaction temperature and HRP dosage on syntan were discussed. The optimum performance of GA-SHBS was got when the molar ratio of GA: SHBS is 1:1 in the presence of 5 mg HRP at 35°C in pH 7.0. The chemical structure of copolymer was characterized by means of FTIR and NMR. The mechanism of polymerization between GA and SHBS and binding mechanism between syntans and leather fibers were proposed. The copolymer was used in making leather as tanning agent and retanning agent respectively, applied results showed that the Ts of tanned leather can reach to 75.6°C, re-tanned leather has the merits of good uniformity, fullness, softness, fine grain and non-plastic feel etc. Syntan also has good permeability, excellent tanning performance and dyeing effect.