A finite element analysis model of 20-node isoparametric elements was established by the commerce FEM software ABAQUS based on the of fracture mechanics theory. The crack at the bottom of the semi-rigid base course was numerically analyzed, and the crack extension life was evaluated by P-M rule and Paris formula. In the course of base crack extension the change of the stress intensity factor (SIF) (K1 and K2) was discussed, and the effect of pavement parameters on base fatigue life was studied. The results reflected that the law of pavement could provide scientific basis for anti-crack design of highway engineering.