Abstract: This paper introduced the ultrasonic inspection technology for the nuclear power plant steam generator（SG）shell and cone butt welds. By means of sound beam angle correction and sound path localization method, the problem of defect location in SG cone inclined plane scanning was solved, and the calculation formula for defect location was derived. The changes of probe angle resulted from the sound velocity variation and the signal location at the depth direction of two materials with different acoustic characteristics are compared, and the signal detected was analyzed by adjusting the sound velocity. It is found that the signal was generated by the weld structure, and the error in locating the defect depth resulted from the difference of sound velocities between the standard test block and the reference test block was solved.