According to the structure characteristics of rocket case's shape, the ultrasonic immersion testing method was adopted to detect the internal and surface flaws accurately. Different methods were used in detecting different part of rocket body. By extracting the work piece's effective echo signals, and then adopting the ultrasonic C-scan imaging technique, the flaw's image of rocket case was obtained. Test results show that the method could detect the work piece without blind spot or omission and make the defected position be accurate. Thus, it could offer an effective testing method to defect the flaw of the rocket case with a kind of structure.