Abstract: Aiming at the problem that stiffness coefficient of bonding interface is difficult to be accurately measured under the condition of nondestructive, a method using ultrasonic to test stiffness coefficient have been proposed. Mathematical model of the ultrasonic testing stiffness coefficient for multilayer medium has been established by the reflection and transmission of acoustic wave propagation in layered medium and the bonding interface spring model. Ultrasonic reflection spectra under different bonding interface have been obtained when the substrate material is steel, cast iron, aluminum alloy, and the coating material is Al₂O₃ ceramic obtained by different spraying processes. The simulation results show that the resonant frequency is periodic on the condition of the debonded or perfectly bonding interface, but the periodicity is different. When the interface is weakly bonded, many resonant frequencies emerge and move to high frequency with the increment of stiffness coefficient. The lower resonance frequencies move faster than the higher ones. The relations between resonance frequency and stiffness coefficient have been established. The variation rules of resonance frequency with the substrate and coating impedance are as follow: the resonance frequencies increase with the increasing of the substrate impedance, and decrease with the increasing of the coating impedance in the same stiffness coefficient. The relation between stiffness coefficient and resonance frequency is fitted using the exponential function. The function including the substrate and coating impedance, stiffness coefficient and resonance frequency was obtained in weakly bonded interfaces. The results will provide theoretical support to ultrasonic testing of the coating composites with weakly bonded interfaces.