The sensitivity of lateral shear interference is quantitatively discussed and the model of average sensitivity is presented, it can explain the theory of lateral shear interference completely. The important result has been obtained that if a tested wave-front is to be reconstructed correctly, we had better use a variable shear interferometer to test it, that is, using more than one shear to test the same wavefront. Then, the tested wavefront can be solved with a new method-Fullprocessing of Lateral Shear Interferograms.