A high-accuracy optimization method for beam shaping, the marginal phase correction, is presented for correcting the phase of the diffractive optical element (DOE) designed by ray tracing to generate a ring-shaped beam. Using an instance of converting a Gaussian beam into a ring-shaped profile, the shaped results of DOEs designed by this new approach, Gerchberg-Saxton (G-S) algorithm and improved Gerchberg-Saxton algorithm are compared. The compared results show that the Gerchberg-Saxton algorithm simultaneously brings about high diffraction efficiency and large root-mean square value and maximum error; the improved Gerchberg-Saxton algorithm can effectively decrease the root-mean-square value and maximum error but also drop down the diffraction efficiency too much; this new approach can balance the diffraction efficiency, root-mean-square value and maximum error to obtain the least root-mean-square value and maximum error upon the slight decrease of diffraction efficiency. The shaped ring of the beam by this new method approaches a perfect ring.