In order to extend the service life of highway and improve the construction and maintenance level of long-life composite pavement, the calculation method of the estimated life of the pavement is carried out based on the Weixu expressway. The stress conditions of the asphalt overlay of composite pavement are simulated by finite element software, the reasons for the negative value of the stress intensity factor are analyzed, and a method of coupling several different types of stress intensity factors into an equivalent stress intensity factor is proposed from the perspective of energy. Based on the Paris formula, a new method is proposed to calculate the cracking age of the overlay using the stress intensity factor of the overlay in the finite element model. The results show that: 1) The good bonding between the asphalt overlay and the cement concrete slab inhibits the bending and tensile deformation of the bottom of the asphalt overlay, so the pavement is not prone to tensile reflection cracks. 2) As the load moves from right to left, the stress intensity factor of shear load is distributed symmetrically, and the crack experiences two shear actions in opposite directions. 3) Based on the characteristics of the Paris formula and the finite element model, the cracking years of the five pavement structures are calculated, and the results are more consistent with the actual situation.