Abstract: The formation mechanism of creeping waves on the inner surface of thick pipes was studied. The laser ultrasonic visualization technique was conducted for observing the propagation of ultrasonic waves on the thick pipes when a straight probe was excited on the outer surface. It was found that the straight probe excites the longitudinal waves with high energy and the shear waves with low energy, and that the creeping waves are formed when the longitudinal waves are incident on the inner surface with a critical angle. Moreover, the mode transformations occur when the longitudinal waves and the shear waves are incident on the inner surface. This study provides experimental references for nondestructive testing the inner surface of thick pipes by using creeping waves.