A novel technique is proposed based on Fourier modal expansion and least-square fitting to reconstruct a two-dimensional wavefront from phase difference resulting from large shear. By using Fourier expansion, the Fourier coefficients of two-dimensional wavefront are computed from two one-dimensional difference wavefronts which are acquired by shearing interferometer. A recipe for data pre-processing is presented to solve information leakage resulting from large shear. This technique is applicable to situation where the shear amount is larger than one sampling interval, and can alleviate the limitation on the shear amount imposed by conventional algorithm. Investigation into the influences of shear amount and noise level on reconstruction accuracy is made.