Abstract: A method for measuring the height of specific defects inside the weld was discussed, which can be used for plate butt welding seam detection. This method adopted the radio frequency function of A-type portable ultrasonic detector and detected it in a dual-crystalline pitch-catch mode. By recognizing the upper and lower endpoint diffraction signals of the defect, the height of the defect itself was calculated. The proposed method was used to detect the plate butt weld verification test block and compared with anatomically verification. The test results show that the proposed method can effectively detect the height of specific defects inside the weld, with a detection accuracy of ±1 mm.