Recently, quantitatively nondestructive testing (QNDT) is becoming an accepted concept in some industries and scientific research areas. The combination of shearing speckle interferometry (SSI) and mechanical models is employed to quantitatively estimate defect characteristic parameters (DCP), such as coordinates, size, embedding depth, etc. However, quantitative calculation of DCP relies on the actual displacement slope in the mechanical models, but the slope in SSI is represented by the difference of displacements between the two neighboring points with a distance, i.e. the shearing amount. This leads to a deviation in calculating DCP. This paper will investigate the deviation of the relative displacement and derivate displacement introduced by the shearographic approximation in cases of two deformation models, one is a thin circular plate and the other a spherical pressure shell under the pressure loading. Two kinds of defects, a cavity and a crack, are embedded in the structures and their deformations are calculated by FEM.