The liner is the middle layer of the solid rocket motor bonding propellant, and its curing state affects the bonding quality of the interface. Therefore, online monitoring and judging the curing state of the liner is the premise to ensure the engine charge and safety quality. In this paper, using laser ultrasound under the ablation mechanism as a means to study the monitoring method and curing state characterization technology of butylated hydroxy lining during the curing process, the online monitoring of the curing process of the lining is described in detail. The time-domain and frequency-domain bispectral characteristics of the echoes are analyzed emphatically, and the propagation time, velocity and nonlinear harmonic third-order spectral peaks of the characteristic waves of the laser ultrasonic longitudinal wave in the lining of the multilayer bonded structure are extracted as the characteristic quantities. , which characterizes the state change law of the liner curing process. The results show that in the time domain information of the characteristic wave, the progress of the curing reaction will lead to a sequential decrease of the arrival time of the characteristic wave between different states by 0.02 µs, and the propagation sound speed of the laser ultrasonic longitudinal wave in the lining layer is successively increased by about 40 m/s; In the frequency-domain bispectral information, the third-order spectral peaks of nonlinear harmonics gradually increased with the progress of the curing reaction, and the ranges of different states were 0.1167, 0.9799, and 0.6360in proper sequence. In conclusion, the online monitoring experiments and laser ultrasonic parameters in this paper can effectively monitor and characterize the curing state of the lining, respectively.