Abstract: Fatigue is a common phenomenon in welded structures, therefore prediction of fatigue life of welded joints in-service is an on-going and unsolved challenge with conventional linear ultrasonic test method. In this paper, nonlinear ultrasonic distortion effect of the crack during the initiation and propagation process in different fatigue cycles of the aluminum alloy joints was studied and the curve of nonlinear coefficient and fatigue cycles was established simultaneously. The pulse-inversion technique was used for signal processing. The experimental results showed that the harmonic wave amplitude of S-N curves processed by pulse-inversion technique is increased one time compared with that by filtering module. Therefore, the pulse-inversion technique can improve the detection capabilities of nonlinear ultrasonic test technique. Experimental results showed that the S-N curve changes slightly in the fatigue crack initiation stage, then increases rapidly with the increase of fatigue cycles, and it begins to decrease when fatigue crack extends to the macroscopic crack. The experiments of microstructures verify the precision of curve. It can be concluded that the nonlinear ultrasonic technology base on the pulse-inversion can be effectively used to predict the fatigue life of aluminum alloy welded joint.