Abstract: In artificial composite materials, the detection of sub-wavelength defects is still difficult.This thesis established two-dimensional aluminum plate models based on finite difference properties method (FDTD) to study the ultrasonic characteristics of sub-wavelength defects. It is found that when the size and arrangement position of the defect change, the waveform of the ultrasonic reflection signal changes accordingly, which can be used to extract the feature from the reflected echo to determine the size and arrangement position of the sub-wavelength defect.