Abstract: Circumferential ultrasonic testing with transversal wave contact technique on cylindrical interface is different from that on planar interface. The curved interface will make some impact on the reliability of ultrasonic testing. The authors set up a geometric model of ultrasonic testing on circular interface by acoustic theory in this paper, and analyzed the impact of circular interface on such aspects as ultrasonic coupling, soundfield structure, parameters of probe characteristics, echo characteristic of area flaw, positioning and quantification of defect, and so on. Experimental study is also made by processing comparison samples of circular interface to validate the theoretical model. Based on this, some appropriate measures are advanced to improve the reliability of ultrasonic testing on circular interface, and these measures have achieved good results in practice.