A two-stage process based on steam explosion pretreatment and alkaline solution post-treatment was applied to fractionate Lespedeza cyrtobotrya stalks into cellulose, hemicelluloses, and lignin. The characteristics of the lignin fractions permit the determination of the influence of incubation time (2-10 min) on the clean fractionation of lignocelluloses. The results indicated that steam explosion at 2.25 MPa for 2-5 min significantly enhanced the lignin fractionation in alkaline solution from 0.47% (raw material) to 16.36-17.13%. However, the repolymerization reactions were extensively conducted at oversevere conditions (6 and 10 min) and partially led to the lower yield of lignin fractions. The cleavage of beta-O-4 linkages was determined by H-1 NMR as increasing incubation time from 2 (11.1%) to 10 min (7.3%). However, the molecular weight of the lignin fractions was gradually increased from 1185 g/mol to 1816 g/mol. These phenomena demonstrated that depolymerization reactions were accompanied with comprehensive repolymerization reactions with the severity in the steam explosion process. It was also found that steam explosion pretreatment at low severities increased the surface area of the isolated lignin fraction. Consequently, this simple and effective process is useful for the further utilization of lignin to produce high-value chemicals.