The interfacial debonded cracks are liable to occur in the stress relief boot of solid missile motor. In order to determine the allowable debonded depth in different storage period, a method to analyze the stability of the interfacial crack is presented. Under the circumstance of internal pressure and axial acceleration loading, the three-dimension finite element models of the motor grain with interfacial debonded cracks were established, and several interfacial debonded cracks were preset along the stress relief boot between cladding and propellant. Thus three-dimension singular crack elements at the tips of fore and rear stress relief boot were established to simulate the debond propagation. Along with the interfacial debonded crack propagation, the stress intensity factors of the crack tips in different storage period were respectively calculated to prejudge its stability. The method and conclusions will be helpful in using solid missile motor with interfacial debonded cracks.