Phase retrieval is one of the most important steps for coherent X-ray diffractive imaging (CXDI), Which uses the oversampled far-field diffraction pattern for phase retrieval iterative algorithm in order to achieve the lost phase information. Here we used a small nonperiodic 2D digital image as the object for studying the effect of the oversampling ratio and obtained the optimum oversampling ratio of 3—7 when the iteration times was 1000. We also added random noise to the diffraction pattern to examine the applicability of this approach to real data. We found the reconstruction failed when the signal to noise ratio is less than 10. As the reconstruction process may fail when the twin image or stochastic shift appears in the reconstructed image simultaneously, we explained why these phenomena come into being, and found methods to overcome the difficulty effectively.