Abstract: This paper combines ultrasonic immersion method and TOFD technology,and applies it in the detection of 9 mm wall thickness pipe ring weld.First of all,the numerical simulation of immersion TOFD (I-TOFD) detection for thin walled pipe girthed weld numerical modeling was performed by finite element method using ultrasonic simulation software Wave 3000.The simulation shows that the longitudinal wave inspired by source after refraction in water-steel interface,continues to spread along under the surface of water-steel interface,and part of the side lobe goes back to the water again and is to be received by the receiving probe.We call the wave as the lateral wave of I-TOFD.Then,the new lateral wave and the bottom echo can be used as reference in actual detection,and the detection accuracy is very good and absolute error is less than 0.2 μs.