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Lignin is recommended to a tempting alternative precursor of petroleum for fabricating carbon nanofibers (CNFs) due to its high carbon content, low-cost and renewable resources. However, the property of lignin-based carbon nanofibers (LCNFs) is inferior owing to the heterogeneity and 3D-network structure of lignin, which hinders its application in supercapacitors. The latest developments in fractionation technology have shown great potential for overcoming the aforementioned shortcomings. However, most of fractionation methods mainly rely on expensive chemicals and complex reaction process, such as enzymes, multiple solvents, membranes, and dialysis tubes. Herein, we proposed a controllable and effective strategy to fractionate lignin by only changing the ratio of ethanol/water (V/V) as mixture solvent. This gradient extraction method effectively removed the part of lignin with small molecular and branching structure, thus selectively getting the fractionated lignin with high molecular weight, narrow polydispersity index, and good linear structure. Fortunately, when the ratio of ethanol/water was 6:4, the corresponding LCNFs (LCNFs-L60) was obtained with large specific surface area, independent filamentous morphology networks and excellent electrochemical property. Its specific capacitance was up to 405.8 F/g. This way features controllable and sustainable for preparing high-quality LCNFs supercapacitors. [Display omitted] • The high linear structure of sugarcane bagasse lignin was obtained through different solvent ratio fractionation way. • The heterogeneity of fractionated lignin was decreased by screening large molecular weight sugarcane bagasse lignin. • L60 with more β-O-4 structural units and smaller homogeneity was more suitable for fabricating high performance LCNFs. • The LCNFs-L60 showed independent morphology, uniform diameter, large surface area and excellent power storage capacity. • This method was an effective strategy for preparing low-cost and high-quality LCNFs supercapacitors electrodes. [ABSTRACT FROM AUTHOR]