In order to deeply investigate the pyrolysis mechanism of lignin, its pyrolysis process is simulated with vanillin, vanillic alcohol and vanillic acid being selected as monomeric model compounds of lignin. In the investigation, the density functional theory is employed to analyze the dissociation sequences of chemical connection bonds, and four reaction paths, namely, Path 1 - Path 4, are designed. Both Path 1 and Path 2 generate guaiacol  but their intermediate processes are not the same, while Path 3 and Path 4 generate catechol and phenol with priority processes respectively. The results show that (1) the methyl in the side chains of the model compounds first dissociates with CH4 being released, and the dissociation sequences of the substituents on C1 depend on their ability to withdraw electrons; (2) in the pyrolysis process, it is difficult for the substituents on C1 to dissociate directly, but prior dissociation occurs instead with the help of outside factors; (3) it is possible for the target products of three kinds to be obtained in the priority sequence of catechol, phenol and guaiacol; (4) from the view of dynamics, Path 2 and Path 4 are superior to Path 1 and Path 3 in terms of dynamic progress; and (5) whenever the target products of the same kind are made,  it is always the easiest job for vanillic alcohol, and then vanillin, whereas vanillic acid is always  the most difficult matter to be dealt with.