Abstract: A kind of defect was found at the root of aluminum alloy T-butt joint. By analyzing the appearance characteristics, metallographic structure and fracture morphology of the defect, it is determined that the defect is a kind of oxide inclusion. The alumina film at the back of the groove is softened and deformed under welding heat, the molten pool overflowing from groove gap fluctuates and entraps the alumina film, and the alumina film is elongated under the shrinkage of the molten pool to form the oxide inclusion during the cooling process. The welding quality of the aluminum alloy T-butt joint is obviously improved and the defect disappears by adjusting the groove gap to 3-4 mm and adding permanent gasket, or adjusting the groove gap to 0 mm.