The limit state function of the interface debonding between the track slab and the CA mortar layer was established based on the mechanism of interface damage. A finite element model (FEM) of the CRTS II slab ballastless track was established, and a cohesive zone model (CZM) was used to model track slab and CA mortar joint. The first four moments of the limit state function were solved based and the reliability index was calculated based on the FEM and method of moments. Under the action of overall temperature rise, the influence of temperature gradient on the debonding reliability index was analyzed under different positions of track slab. The results show that overall temperature rise has a great influence on the debonding reliability index. Under the action of overall temperature, the debonding reliability index decreases with an increase in temperature gradient. The slab center is easier to debond under the action of extreme positive temperature gradient.