As the morphology of an ultra small aperture and the assembling error will affect the quality of wavefront diffraction in development of a point diffraction interferometer, this paper explores an evaluating method for wavefront diffracted quality from the ultra small aperture based on the phase retrieval. The testing equipments for wavefront diffraction was designed and the evaluating testing experiment for the wavefront diffraction were performed. To verify the effectiveness of the method based on the phase retrieval, the captured and reconstructed images were compared, and the obtained amplitudes with the theoretical calculation and experimental test were also analyzed. The results show that the difference between captured images and reconstructed images is only 0.32%, which verifies that the proposed method is feasibility. Moreover, the interferometric measurement result was compared with that of the phase retrieval proposed in this paper, and the results show that the difference is only 0.001 4λ RMS(Root Mean Square).