A novel two-dimensional phase unwrapping method is proposed. In this idea, the phase wrapped is propagated through an optical diffraction calculation as an input optical field and transformed into the intensity distribution. Then the phase retrieval algorithm is engaged in order to find the best fitting Zernike polynomials for the unwrapped phase based on the diffraction intensity. Because of the maturity of phase retrieval theory, the uniqueness and accuracy of phase unwrapping can be guaranteed. As a result, the phase noise can be remarkably reduced by intensity filtering. The validity of this algorithm is verified by experiment.