Abstract: In order to characterize flaws and localize failure for water immersion ultrasonic testing, the copper bar was taken as an example to show how to solve this problem in the present article. The water immersion ultrasonic test was employed and the displays of flaws were analyzed. Through combining the test with the fluorescent penetrant testing and metallurgical examination, the nature and location of flaw were clearly explained, which confirmed the liability of the water immersion ultrasonic testing for the copper bar and provided a useful reference for the analysis of water immersion ultrasonic testing.