The lining is an important part of connecting the solid rocket motor shell and the propellant, and its bonding state determines the integrity of the propellant-liner-shell bonding interface, which further affects the safety and reliability of the solid motor. According to the mixed state and discontinuous impedance characteristics of the lining bonding structure, the propagation law of laser ultrasound in the lining structure is studied, and the experimental device is set up. By means of ultrasonic wave excited by ablation mechanism, the propagation time and relative sound attenuation of ultrasonic wave in the lining are extracted, and the ultrasonic transmission method is used to characterize the curing process of the lining. A calibration time difference method for solving the longitudinal wave transit time in the lining is proposed, and the relative sound attenuation model of discontinuous impedance is established. The experimental results show that the transit time obtained by the calibration time difference method and the sound attenuation calculated by the relative sound attenuation model can well characterize the curing process of the lining.