Abstract: AS -AQ pulping of eucalyptus (Leizhou No.1) wood and the structural change of lignin during pulping were studied. It was found that the lignin in eucalyptus has guaiacyl-syringyl-p-hydroxylphenyl (G -S -H) structural characteristics. The ratio of S:G is 1.25:1. The Mw is 6 784,Mn is 5585.The guaiacyl groups in lignin was dissolved faster than syringyl groups in lignin during AS -AQ pulping.The sulfonation reaction and delignification rate of lignin in secondary walls(S) were slower than that in cell corner (CC) and compound middle lamella (CML) during AS -AQ pulping, so that residual lignin content in AS -AQ pulp was higher. Condenzation of lignin was occurred at the end of cooking, so that the permanganate number of pulp was raised and the brightness of pulp was reduced.