Abstract: Extraction of lignin from sulfite pulping waste liquor was investigated. The extraction rate of lignin from black liquor by desulfonation increased in comparison with that without desulfonation. Then the influence of 15% H2SO4，precipitation time，precipitation temperature on the separation of lignin from black liquor by desulfonation was studied through 53 orthogonal experiments. The result indicated that the optimal condition of purifying lignin was as follows：the proportion of black liquor and 15% H2SO4 was 1∶2 and cooking at 90℃ for 1.5h. Lignin obtained from desulfonation of black liquor，had high purity，high molecular weight，good-heat stability and plasticity through the analyses of IR spectrum，GPC，TG-DSC and Torque curve.