Abstract: This paper describes the ultrasonic inspection technology for butt joint of unequal thickness in the structural modules. When the acoustic wave obliquely enters into or reflects on the angular surface, the location of defect must be modified in the method of geometric analysis because of the technical difficulty for location. The last part briefly summarizes the factors of influence on the positions accuracy. So, if we want to locate the defect correctly, we need qualified examiners, devices and appropriate methods.