To improve the reactivity of kraft lignin, an alkaline treatment of kraft lignin was performed in the condition of different time, temperature and concentration of NaOH. Differential ultraviolet spectrophotometry and GPC were used to evaluate the content of phenolic hydroxyl and distribution of the molecular weight. The structure changes of lignin were characterized by FTIR. The results showed that the optimal reaction condition for alkali-catalyzed hydrolysis of kraft lignin was 7.5% NaOH, reaction at 170℃ for 6 hours, and the content of free phenolic hydroxyl increased after alkaline treatment. The molecular weight of lignin was obviously lower than the original. The polydispersity also decreased. It indicated that the range of molecular weight of lignin has a tendency to be narrow. The cleavage of aryl-ether bonds and ether bonds linked with the units of lignin may be responsible for the increase of phenolic hydroxyl and degradation of lignin.