In this study, we examined the nonlinear ultrasonic effect of closed microcracks in industrial plates and derived the relationship between the nonlinear coefficient and the characteristic parameters of closed microcracks by analyzing the contact nonlinear ultrasonic theoretical model. Firstly, we established the finite element mesh model of closed microcracks in an industrial aluminum plate by simulation analysis and excited the S1 mode Lamb wave in the plate by the wave structure loading method based on the dispersion curve of the material’s nonlinear matching. Secondly, we set up sensor nodes to collect the signal after the interaction between the S1 mode Lamb wave and the closed microcracks. Finally, we preprocessed the signal to analyze the rule of the change in the level of interaction between the closed microcracks and the S1 mode Lamb wave depending on the characteristics. Experimental results show that the second-harmonic amplitude and nonlinear coefficient decrease with increasing crack width for the closed microcracks, but increase with increasing crack length. These results confirm the effectiveness of the contact nonlinear ultrasonic theoretical model.