Abstract: Elaborated here in the essay is the ultrasonic detection method——the deformed transversal wave angle beam process as used for inner flaw-detecting of extra-heavy-wall steel tube. With the probe incident angle selected within the range smaller than the first critical angle, the refracted longitudinal wave is obliquely shot on the outer surface of the tube to produce deformed transversal wave so as to detect the tube’ s inner longitudinal flaws. Elaborated are the main aspects of the process as being used to detect extra-heavy-wall tube flaw, covering determination of the probe incident angle, adjustment of the detection sensibility, and wave shape discrimination, etc. The effectiveness of the detection process is verified after being actually used to inspect the Ф121 mm×36 mm tubes.