As the major flaw of highway tunnels, lining cracks of highway tunnels is increasingly seriously. In order to study the mechanism of crack propagation in highway tunnels under loading, this article conducts numerical simulation on the crack propagation with discrete element software PFC, demonstrating the disaster mechanism of crack propagation in highway tunnels,and establishing the corresponding prediction models. The results showed that:integral safety of lining is subject to pre-existing flaws, and the positions of pre-existing flaws are generally relatively more dangerous in lining; the depth of pre-existing flaws plays an important role in the timing of crack coalescence,i.e.,the deeper the pre-exsiting flaws are, the less time is needed to achieve crack coalescence in the lining; the depth of pre-exsiting flaws has a great influence on the variation of crack opening, i.e. the deeper pre-exsiting flaws are, the increasing opening those fracture apertures will be, and there is an obvious abrupt change when the flaws coalesce.