The failure reasons of flaring test for welding seam of hollow extrusion profile for aluminum alloy floor were analyzed by composition measurement, metallographic, transverse tensile test and SEM observation. The analysis results show that the main failure reason of flaring test for welding seam of hollow aluminum extrusion profile is that there is a bonding interface at the welding seam position, which is not continuous with the matrix structure of aluminum alloy. Under the action of external force in the flaring test, cracks germinate at the discontinuous position of bonding interface and aluminum alloy profile which causes the failure of the flaring test. From the transverse tensile test results of the failure position and the normal position for the flaring test, the tensile strength and yield strength decrease significantly, and the elongation decreases to 2.3%, which is 8% lower than the required value. In addition, the fracture of the tensile specimen is flush, which is typical brittle fracture. However, the macroscopic morphology of the transverse tensile fracture at the failure position in the flaring test shows a layered and discontinuous phenomenon. Through SEM observation, there is no dimple feature at the failure postion in the welding flaring test, and there are obvious cracks and cavities.