The esterification and polymerizing reaction of rosin were studied.At first,by reacting of rosin and benzyl alcohol,rosin benzyl ester was prepared in the toluene with Lewis acid as catalyst.In esterification reaction,we selected the optimum conditions as follows:reaction time was 90 min,weight of catalyst was 4 g,ratio of raw material was 1∶1.05(mol),reaction temperature was 100～114 ℃.The method of purifying of the ester was discussed.It was purified by column partition chromatography with anion exchange resin as immobile phase,and with toluene as mobile phase.The purifying method achieved good results.Besides,the polymer of the rosin benzyl ester was synthesized by catalyst,and with stirring in the closed flask for five hours at 50 ℃.IR and UV of the polymer were analyzed,and its solubility and molecular weight was determined.Its solubility was greater in the polar organic solvent than the nonpolar organic solvent.Its molecular weight was 1 340 g/mol.Average polymerization degree was 3.42.