Abstract: The finite element model of nonlinear ultrasonic testing was established by using ABAQUS finite element software. The influence of the size of kissing bond defects on the nonlinear coefficient of ultrasonic was calculated. The calculation results show that the amplitude of the fundamental wave and the amplitude of the second harmonic gradually decrease with the increase of the size of the interface kissing bond defects, while the ultrasonic nonlinear coefficient increases monotonously. The nonlinear ultrasonic testing of five AZ31 magnesium-aluminum alloy bonded structural specimens with different weak bond defects was carried out. The results show that the test results are consistent with the finite element calculation results, indicating the effectiveness of nonlinear ultrasonic testing method for kissing bond defect in bonded structure.