AbstractEven though lignin carbon fiber has been sought after for several decades, the poor mechanical performance remains to be a major barrier for commercial applications. The low mechanical performance is attributed to the heterogeneity of lignin polymer. Recent advances in fractionation technologies showed the great potential to reduce lignin heterogeneity, but current fractionation methods often depend on costly chemicals and materials such as enzymes, organic solvents, membranes, and dialysis tubes. Here, a new non‐solvent strategy was developed to fractionate lignin by autohydrolysis. By using only water, lignin was efficiently fractionated into water‐soluble and ‐insoluble fractions. The latter fraction had increased molecular weight and uniformity and resulted in more β‐O‐4 interunitary linkages as analyzed by size‐exclusion chromatography and 2D heteronuclear single quantum coherence NMR spectroscopy, respectively. In particular, the water‐insoluble fraction significantly enhanced the mechanical performances of the resultant carbon fibers. Mechanistic study by differential scanning calorimetry (DSC) revealed that the miscibility of lignin with guest polyacrylonitrile molecules was improved with the reduced lignin heterogeneity. Crystallite analyses by XRD and Raman spectroscopy revealed that the crystallite size and content of the pre‐graphitic turbostratic carbon structure were increased. The fundamental understanding revealed how lignin fractionation could modify lignin chemical features to enhance the mechanical performance of resultant carbon fibers. The autohydrolysis fractionation thus represents a green, economic, and efficient methodology to process lignin waste and boost lignin carbon fiber quality, which could open new horizons for lignin valorization.