Enhancing the efficient isolation of proto-lignin from lignocellulose using sustainable, environmentally friendly solvents has consistently been a key focus in the lignin-first biorefinery strategy for the past years. At the same time, achieving efficient extraction of highly uncondensed active lignin under acidic conditions has posed an insurmountable challenge. In this study, we develop an L-cysteine-assisted binary acidic cosolvent that efficiently separates the uncondensed lignin under 80 ∼ 90 ℃ treatment, resulting in a lighter color and a recovery yield of up to 60 %. The corresponding delignification rate reached 82.6 %. The obtained lignin exhibited an ideal retention rate (maximum 84.5 %) of alkyl aryl ether bonds (β–O–4) relative to the native lignin in raw material. The resulting residue was free from cellulose decomposition and yielded over 90 % cellulose with a surface-esterified structure. This study establishes a one-step L-cysteine-assisted stabilization methodology that overcomes the challenges of lignin condensation and allows for the fractionation of lignocellulose into highly uncondensed lignin and high-quality cellulose, thus offering new solutions for biorefinery.