Abstract: The residual lignin of triploid clones of Populus tomentosa from kraft pulp (KP) and sulfite pulp (ASP), as well as the lignin in the waste liquor was isolated and analyzed by Fourier transform infrared (FT-IR) spectrum and ¹³C nuclear magnetic resonance (NMR). Results showed that the condensed structure and the connection between lignin and semi-cellulose in ASP were less than those in KP.A large number of structures of benzene, methyl and methylene groups, but rare aralkyl ether linkage existed in the waste liquor.There were more carboxyl groups, condensed guaiacyl units and guaiacyl primary alcohol in the waste ASP liquor than those in KP waste liquor.