When detecting microcracks based on the photoacoustic nonlinear frequency-mixing method, the characteristics of nonlinear acoustic signals are affected by the crack contact state, which is affected by its initial width, wall morphology, modulation intensity, and other factors. This results in a change in the detection process of the nonlinear mechanism of the interaction between acoustic waves and microcracks. Therefore, in this paper, a two-dimensional physical model is established to analysis the nonlinear frequency-mixing process of crack detection, and the penalty method is used for the enforcement of contact constraints. The experimental result of the model are compared and analyzed with existing experimental results to verify the applicability of the model. The acoustic signal characteristics are investigated under different contact states, and the nonlinear mechanism is affected by the change of the crack contact state, which provides a theoretical reference for studying the photoacoustic nonlinear detection of microcracks.