Abstract: Ultrasonic testing was used to detect the large thickness (more than 30 mm) and narrow-gap (less than 20 mm) weld in a titanium alloy product. Under different detecting conditions, the results were found to be different, and a defect whose wave amplitude was located in area III above rejection line was even found. This paper further analyzed the causes of the diference, and the defect was ultimately identified as non-dangerous defect by dissection and radiography of the weld. This paper can work as a reference to those who are concerned.