A wavefront reconstruction technique by two-step generalized phase-shifting interferometry （GPSI） is proposed and verified by optical experiments. According to the statistical characters of the diffractive field, a blind extraction algorithm is developed, by which the unknown phase shift value in interferometry can be successfully extracted. The proposed method can retrieve the real amplitude of object wave field by using two interferograms, the recorded object and reference wave intensities, and an extracted phase shift, without the assistance of additional Fourier spectrum analysis or digital filtering. In optical experiments, the feasibility of this method for both the indirect object （digital images cast on a spatial light modulator, SLM） and the direct three-dimensional diffused object inputs has been discussed and investigated in detail, and the experimental results have convincingly verified the usefulness and effectiveness of this scheme in practice.