Lignin is extracted by an improved organosolv method in the presence of different stabilization reagents including formaldehyde, formic acid, and methanol. The structure and thermochemical properties of the extracted lignin were characterized by gel permeation chromatography, Fourier transform infrared spectroscopy, heteronuclear single quantum correlation nuclear magnetic resonance, and thermogravimetric analysis techniques. During the extraction pretreatment, the fragmentation of lignin was suppressed via the addition of formaldehyde. The characteristic structure of natural lignin was retained by inhibiting the beta-O-4 bond from cleavage. The fast pyrolysis of the extracted lignin was analyzed by the pyrolysis-gas chromatography/mass spectrometry technique. Compared with the formaldehyde-stabilized lignin, unstabilized lignin appeared to be more thermally resistant to volatilization. Because of the stabilization effect of formaldehyde, the pyrolysis yield of condensable volatiles was tripled; meanwhile, less char was formed. The improvement in pyrolysis performance could be ascribed to the retention of original aryl-ether bonds during delignification, and therefore, the unfavorable condensation of the C-C bond was largely inhibited.