Abstract: In view of the problem that the traditional linear ultrasonic detection technology cannot detect fatigue micro cracks, a nonlinear ultrasonic guided wave detection technology for fatigue micro cracks is proposed. Firstly, the Lamb wave mode in the aluminum plate was determined by comparing the detection of signal time-frequency distribution with the calculation of dispersion curve, and then the experimental parameters were optimized and a single mode Lamb wave was excited in the plate. Secondly, the nonlinear response conditions of the mode Lamb wave were determined by the experimental method to determine the practicability of the mode in excitation of micro-crack fatigue damage. Thirdly, the characteristic parameters of nonlinear ultrasonic detection were extracted and the relationship between the characteristic parameters and fatigue cycle number was analyzed. The results show that the dispersion characteristic of Lamb wave in aluminum plate leads to a large fluctuation of fundamental frequency amplitude, and the nonlinear coefficient cannot reflect the true nonlinear response of micro cracks. The second harmonic amplitude increases with the increase of fatigue cycle in the early stage of fatigue damage.