Abstract: This paper introduces the structure of the control rod drive mechanism pressure housings welds (CRDM) of the third generation CEPR unit. In view of the material and microstructure of the weld, the requirement of the lower weld of the control rod drive mechanism for ultrasonic testing technology was put forward and experiment to verify the technology was designed. The results showed that the ultrasonic testing method could detect a defect of its height being equal to or greater than 3 mm.