Abstract: According to RSE-M standard, the main bolts of reactor pressure vessel (RPV) in nuclear power plants need to be inspected by ultrasonic regularly. In order to ensure the sensitivity of ultrasonic inspection of notches in different depths of stud thread area and rod area, this paper carries out the sound field simulation calculation for the detection process, analyzes the relevant and non relevant signals in data acquisition, and focuses on the characteristics of crack signals to verify the reliability of ultrasonic technology. Combined with the field case, the abnormal signal can be determined effectively by the high reflectivity of 45° S-wave and other detection methods such as eddy current and penetration testing.