Abstract: In the ultrasonic inspection of the big diameter and thick wall tube, owing to the limitation in the first critical angle, the pure transverse wave contacting detecting method is only used for the tube of wall thickness to outer diameter ratio being less than 0.2. Though theoretical analysis, it is proposed, in this text, to use the longitudinal wave oblique probe of the incident angle being 10° to detect the tube in circumferential direction with wall thickness to outer diameter ratio being greater than 0.2. The defect can be differentiated according to the sound pressure reciprocating transmittance (wave height) of refraction longitudinal and transverse wave and the level place on the monitor. Proofed by facts, The method is practical.