Abstract: The efficient separation and extraction of lignin components is one of the key technical problems that hinders the utilization of plant-based biomass resources. Due to their unique and adjustable molecular structures, diazabicyclo solvents show great potential in the application of dissolution and extraction of lignin from plant-based materials. A diazabicyclo-based switchable solvent (SS), composed by 1,8-diazabicyclo5.4.0undec-7-ene (DBU), hexanol, and water, demonstrates exceptional potential in lignin dissolution. Due to its unique and adjustable molecular structures, the SS could dissolve various lignin and its model compounds efficiently even at room temperature. Systematic evaluation revealed an impressive dissolution capacity of 453.2 mg/g for milled wood lignin (MWL) at 25 ℃, far exceeding that of traditional solvents such as methanol, ethanol and deep eutectic solvents. Analysis of Kamlet-Taft solvent parameters indicated that the high hydrogen bond basicity (β) and net basicity of the SS substantially contribute to lignin dissolution. Lignin and its model compounds act primarily as hydrogen bond donors, interacting with the strong hydrogen bond acceptor regions within the SS. The formation of strong hydrogen bonds and non-polar interactions (including π-π stacking) synergistically promote the dissolution lignin in SS. When applied to biomass, the SS system achieved high dissolution rates of 90～93% for lignin extraction from wheat straw and poplar sawdust. Crucially, the extracted lignin retained its structural integrity. Fourier Transform Infrared Spectrometer (FTIR) and Two Dimensional Heteronuclear Single Quantum Coherence Nuclear Magnetic Resonance (2D HSQC NMR) analyses revealed that during the SS treatment the cleavage of carbonyl (C=O) groups and the formation of α-O-R linkages occurred. These α-O-R linkages could be further hydrolyzed into α-OH, which facilities the solubility in solvents. The crucial β-O-4 linkages remained intact, preserving the native aromatic framework. Gel Permeation Chromatography (GPC) further showed that the regenerated lignin had a more uniform molecular weight distribution (PDI of 1.56 for poplar and 1.52 for wheat straw) compared to traditionally extracted lignin. Consequently, the extracted lignin exhibited significantly enhanced solubility in both aqueous and organic solvents. For instance, poplar lignin solubility in water increased from nearly nothing to 205.68 mg/g, and its solubility in diethyl ether surged by 1859% (from 30.25 mg/g to 592.56 mg/g). Wheat straw lignin solubility in diethyl ether similarly increased by 988% (from 45.21 mg/g to 492.05 mg/g). These enhancements are linked to the beneficial structural modifications induced by the SS treatment. In summary, this study highlights the diazabicyclo-based SS as a sustainable and effective solvent for lignin extraction. It efficiently dissolves lignin under mild conditions, preserving the core structural integrity of lignin, particularly the β-O-4 linkages—critical for downstream functionalization. Mechanistic analysis shows that only the α-carbonyl (C=O) and α-ether bonds are selectively disrupted, producing lignin stream with a uniform molecular weight distribution and significantly improved solubility in polar solvents. This study provides a novel technical pathway for the separation, extraction, and high-value utilization of lignin from agricultural and forestry residues. Future research can explore the catalytic conversion and high-value utilization of lignin based on the structural characteristics of SS-lignin. The investigation on comprehensive utilization methods of the carbohydrate components, and the recovery of SS solvents should also be considered to enhance the large-scale application.