The solidification of the solid rocket motor lining is a key step of the solid rocket motor, and its state will affect the final performance of the rocket motor. The curing process of silicone sealant was monitored by laser ultrasonic on-line monitoring platform. This method combines the empirical mode decomposition (EMD) algorithm to process the laser ultrasonic echo signal, and characterizes the curing state according to the changes of ultrasonic propagation velocity and ultrasonic attenuation. The monitoring results show that, as the curing reaction progresses, the propagation speed of the ultrasonic wave increases rapidly, then decreases slightly, and finally stabilizes. The ultrasonic attenuation first decreases rapidly, then decreases slowly, and finally stabilizes. This fully reflects the entire curing process and can determine its semi-cured state, laying the foundation for non-contact online monitoring of the curing state of the lining.