Abstract: With the assistance of ultrasound, the carboxymethyl pachymaran was prepared using twice alkalization method. The influence of reaction conditions of carboxymethylation including different ratio of raw materials (NaOH/Cl-CH2COOH), concentration of media reaction and reaction time on degree of substitution were studied. Then the optimization of carboxymethyl pachymaran by the orthogonal design was investigated, it was found that the optimum conditions were 80%(V/V)of the concentration of ethanol, NaOH/ Cl-CH2COOH(mol/mol)1.5/1, reaction time 50min.