In the preparation of monocyclic benzenes by catalytic pyrolysis of lignin, how to break the bonds of pyrolysis macromolecules into small molecules is the key point for the transformation. In this work, density functional theory was used to explore the mechanism on catalytic pyrolysis of the beta-O-4-type lignin dimer and the effect of H protons derived from solid acid catalysts during the pyrolysis process. The results indicate that C-beta-O bond cleavage is the main pathway for macromolecular breaking. When the H proton attacks the protonation site O13, the bond level of the C-beta-O bond decreases from 0.911 to 0.674 and the dissociation energy decreases from 210 to 179 kJ/mol. Phenol (P1), p-hydroxybenzaldehyde (P2), and p-hydroxyacetophenone (P5) are the main products of the subsequent pyrolysis pathways based on C-beta-O bond homolysis because of their more competitive formation pathways. The attack of H proton on O13 inhibits the formation of phenol, and it does not participate in the subsequent reaction pathways R7-1, R7-2, R7-3, and R7-4. P-hydroxybenzaldehyde (P2), p-hydroxyacetophenone (P5), and ethanol (P3) are produced because of the promotion of the preliminary pathway R7. It is found that H protons can weaken the C-beta-O bond and promote the formation of small molecule compounds by lignin pyrolysis macromolecular components.