The modification of alkali lignin with laccase was investigated. The structural change of lignin was analyzed. The sulfonation reactivity was measured by the content of sulfonic group. The results showed the sulfonation reactivity increased to some extent under the condition of atmosphere pressure, but decreased under the condition of 0.3 MPa oxygen pressure. The analysis of Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) showed the cleavage of various ether linkages and demethylation took place in the structure of lignin to certain extent during modification with laccase, which contributed to the improvement of sulfonation reactivity. Under the condition of 0.3 MPa oxygen pressure, the ratio of s/g (guaiacyl/syringyl) increased after modification, which reduced the sulfonation reactivity of lignin. Simultaneously partial polymerization reaction, such as 4-O-5', beta-5, 5-5 and other reaction in the aromatic ring decreased the activity sites of C-2, C-5 and C-6. Abundant polymerization reaction of alpha-O increased stexic hindrance of C-2 and C-6 in aromatic ring, resulting in low sulfonation reactivity of lignin. (C) 2013 Elsevier Ltd. All rights reserved.