In this paper,based on the theory of fracture mechanics,the safety evaluation of Hook defects in joints of friction stir butt-lap welded aluminum alloy profiles is carried out, and the fatigue test is used to verify it. The metallography shows that the essence of the Hook defect is the result of the upward migration of the overlapping surface under the vigorous stirring action of the stirring head,which appears as a crack.The Hook defect is normalized to a unilateral crack extending into the inside of the weld. The linear fitting of crack growth rate test shows that the threshold value of the stress intensity factor amplitude ΔK,th, is 2.509 MPa·m,1/2,. According to the crack growth conditions,the defect size-stress amplitude relationship curve for evaluating the fatigue growth of Hook defects is obtained.There is a certain error between the fatigue test results and the theoretical Hook defect critical growth stress amplitude. The average error is 9.4%. The overall trend is in good agreement with the theoretical curve. The theoretical formula can be applied to the prediction of the critical stress amplitude of Hook defect fatigue growth,then it can be used to evaluate the safety of hook defects in joints under different working stress amplitudes.