The early detection and monitoring of microcracks in engineering structures play a critical role in preventing catastrophic failures. It has been shown that the nonlinear Lamb waves exhibit a high degree of sensitivity to micro-defects within structures and have found widely noticed in engineering. The second harmonic Lamb wave originates from the contact acoustics nonlinearity effect of the excitation signal with the microcracked surface. The nonlinear information contained in the second harmonic generated by narrowband nonlinear Lamb waves is limited by the frequency of the fundamental wave and is prone to missing effective damage information. Building on this principle, this study systematically investigates the nonlinear responses of isotropic thin plates subjected to three broadband excitation signals with frequency ranges of 200–400 kHz, 300–500 kHz, and 400–600 kHz under varying levels of damage. Additionally, the research examines the nonlinear response of microcracks in anisotropic thin plates under identical damage conditions. The findings reveal that as the severity of microcrack damage increases, the corresponding nonlinear response diminishes. Furthermore, anisotropic laminates exhibit a more pronounced nonlinear response compared to isotropic thin plates under equivalent damage levels. These results provide theoretical insights and technical guidance for the detection and monitoring of micro-defects in composite structures.