When a sandwich structure is loaded in uniaxial in-plane compression the skin over the debonded region may buckle allowing further propagation of the debond, leading to structural collapse. Hence how to detect the shape and size of debonded defect, and to trace propagation trend of the defect are key factor in the process of damage evaluation of subsurface defect. In the paper, a nondestructive testing system is developed based on liquid crystal cell and Wollaston shearing prism. Nematic liquid crystal has the property of modulating the intensity and phase of light, and the intensity will not change during the phase modulating. According to this property, the phase-shifting of digital shearography are performed successfully. The experimental results prove that we can not only perform detection of the defect size and shape in subsurface of sandwich structure, but also trace the further propagation of debonded defect because of local buckle. This indicates that the method of digital shearogrephy will offer an effective measurement tool to research the buckle behavior of debond defect in subsurface of sandwich under uniaxial in-plane compression.