Abstract: β-O-4 is the primary type of linkages among the main lignin structure units. The pyrolysis of lignin dimer model compound of β-O-4 linkage was investigated by using density functional theory B3LYP methods at 6-31G(d,p) level. Three possible pyrolysis pathways were proposed:the subsequent reactions after the homolytic cleavage of C_β-O bond, the subsequent reactions after the homolytic cleavage of C_α-C_β bond and the concerted reactions. The equilibrium geometries of the reactants, transition states, intermediates and products were optimized and the standard kinetic parameters for each reaction pathway were calculated. The formation mechanism of the main pyrolysis products and the effect of temperature on the pyrolysis mechanism of lignin dimer were analyzed. The calculation results show that the subsequent reaction pathways after the homolytic cleavage of C_β-O bond and the concerted reaction pathways (1) and (3) are the major reaction channels, whereas the subsequent reaction pathways after the homolytic cleavage of C_α-C_β and the concerted reaction pathways (2) and (5) are the competitive reaction channels in the pyrolysis process. The main pyrolysis products are phenolic compounds such as guaiacol, 1-guaiacyl-3-hydroxy-acetone, 1-guaiacyl-3-hydroxy-propaldehyde and guaiacyl-formaldehyde. In the pyrolysis process of the lignin dimer, the concerted reactions dominate over the free-radical homolytic reactions at low temperature, whereas but the free-radical reactions prevail over the concerted reactions at high temperatures.