A new method of big shearing digital speckle interferometry is presented. Grating is placed in image space of objective lens, and used as the shearing part. Light wave from reference object and measured object interfered at the image space. The principle of this method is discussed in detail based on Fourier optics, and plane displacement of typical specimen is measured which is a peripheral region fixed block board having symmetrical load. The results show that experimental value and theoretical value are fit well. Some applied example are given. Theory and experience are shown that this method not only have the high sensitivity of holographic interferometry (λ/4), but also have merit of low shockproof demand and in-site measurement.