The damages formed in the early stage of degradation are mainly micro-cracks, and the nonlinear effect between micro-cracks and ultrasound can characterize the damage of materials. In this paper, the influence of the crack parameters on the nonlinear coefficient is analyzed theoretically based on the classical microcrack nonlinear spring model. Then, three-dimensional models of aluminum alloy plates with different crack parameters are established. High-frequency Lamb waves are used for numerical simulations to study the relationships between nonlinear coefficients and the length, width and angle. The experimental results show that as the length of the crack increases, the nonlinear coefficient increases; the width of the crack increases, and the nonlinear coefficient decreases; when three receiving probes are arranged in the Lamb wave propagation direction and the vertical directions, the three-point nonlinear coefficient changes with the angle. The angle of the crack can be roughly judged by the three-point nonlinear coefficient value. The research work is of great significance for the subsequent use of high-frequency Lamb waves to quantitatively detect the angle of micro-cracks.