The chemical structures of four alkali lignins isolated from poplar, fir, straw, and bagasse were investigated. To explore the relationship between the structural units and the thermal decomposition behavior, the system was tested by elemental analysis, Fourier transform infrared spectrometry, thermogravimetric analysis (TGA), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The results indicated that the carbon content of poplar lignin (PL) was higher than that of others. Fir lignin (FL) exhibited the highest guaiacol units, while the other three lignins were abundant in syringol units. The thermal decomposition characteristics and pyrolysis products of the four lignins were influenced by the material structural and composition. The DTG curves showed that the initial temperatures and major degradation temperatures of woody lignins(FL and PL) with complex inherent structures were shifted to the high temperature zoom compared with that of non-woody (BL and SL) lignins. Py-GC/MS analysis showed that guaiacol-type phenolic compounds were predominant pyrolysis products derived from the four lignins. The yield of guaiacol-type phenols could reach 82.87%. Moreover, the BL had selectively on phenol-type compounds with yield of 27.89%. [ABSTRACT FROM AUTHOR]