Abstract: The shear and longitudinal wave detection can not be realized simultaneously in the thick wall steel pipe, whose ratio of thickness and radius is more than 0.2. Aiming at this problem, through the analysis to the detecting principle of thickwall steel pipe, the ultrasonic wave incident angle was calculated, when the shear and longitudinal wave was tangent with internal wall respectively. The acoustic distance and focal distance in the water were determined as well. Thus the detection to the thickwall steel pipe was realized. Based on the theory, a detecting machine was designed to realize automatic and high speed detection to the thick wall steel pipe.