A high yield of lignin was generated when solid alkali (MgO) and active oxygen (O-2 and H2O2) were used in bagasse Cooking as a:pretreatment for lignin conversion. In the present work, the lignin present in raw material, pulp, and yellow liquor was characterized by Fourier transform infrared spectroscopy (FTIR) and heteronuclear single-quantum coherence (HSQC) NMR. The results showed that 95.4% of the lignin was obtained from the raw material and present in the yellow liquor. The syringyl (S/S'/S '') units could be changed completely and removed from the raw material, and a novel G' unit with a carbonyl group was generated through an oxidizing reaction. The phenolic structures had a high reactivity during the cooking process, and the beta-O-4' (A/A'/A '') structures had different levels of reactivity. Moreover, the H unit, P structure, and nonphenolic beta-5' and beta-beta' structures were stable throughout the cooking process.