Abstract: Degradation of material mechanical properties in the early stage is always accompanied by some form of non-linear mechanical behavior, which leads to the generation of high-frequency harmonics. In this paper, the model of randomly distributed micro cracks with length less than half wavelength is established to describe the early damage of material. The ultrasonic nonlinear characterization of the micro cracks is calculated, and the influence of diffraction effect on the ultrasonic nonlinear characterization is analyzed. The results show that the detection signal has obvious nonlinear characteristics when the early damage hardly affects the material property, besides, as the number of cracks increases, the superposition of secondary sources produces more complex frequency components, which significantly affects the non-linear characteristics. The research work provides technical support for early damage assessment of materials.