In this paper,a non destructive testing system is constructed based on laser shearography. By drilling circular blind holes and covering a rubber layer on an aluminum alloy plate,composite bonded specimens with internal defects of different sizes and depths are fabricated. By applying negative pressure excitation,the speckle fringe and out of plane displacement on the surface of the internal defects of the specimen were studied under different negative pressure values. Meanwhile,the finite element calculation is carried out to verify the effectiveness of the non destructive testing system. The results show that the larger the defect size,the greater the order of the speckle fringe and the greater the out of plane displacement from the surface for the same defect depth and negative pressure；the deeper the defect depth,the smaller the order of the speckle fringe and the smaller the displacement from the surface at the same defect size and negative pressure value. After obtaining the accurate acquisition of information on the shape,position,and size of internal defects by the non destructive testing system,the defect depth is further deduced based on the elastic thin plate theory. The accuracy of defect depth measurement increases with the size of the defect,and decreases with the depth of a circular interior defect with a diameter of 20mm and a depth of less than 1mm. This study provides an effective way to detect internal defect depth in composite material bonding structures.