To study the bleaching mechanisms of H2O2 bleaching, H2O2 displacement bleaching and H2O2 /TAED bleaching, three kinds of lignin bleached with above processes were obtained. Three types of treated Populus nigra L lignins structure were identified and the functional groups were quantitied by FTIR. In contrast, the deacetylation and the breakage of ester bonds in H2O2/TAED bleaching system was much more significant. The total carbonyl group of H2O2 bleached lignin decreased 6.6% while displacement bleached lignin and H2O2/TAED bleached lignin decreased 9.2% and 15.8% respectively. The PT bleaching process had stronger oxidability to destroy the syringyl units in lignin. The breakage of ether bonds and alcoholic hydroxyl dehydration in side chain increased after bleaching. In contrast, the stronger destroyed effection on guaiacyl dialkyl ether bond with PT bleaching processes. The conclusions obtained may interpret H2O2 displacement bleaching and H2O2/TAED bleaching systems have better bleaching efficiency than H2O2 bleaching system.