This study proposes a new algorithm for designing diffractive optical elements to solve the issues resulting from the fact that geometric mapping theory does not involve the diffraction effect, which results in an inability to obtain a precise output beam and a serious ringing effect of the output beam. The proposed algorithm sets a parameter for reflecting the diffraction effect. The optimal value of this parameter is determined using an iterative method. The initial phase is modified using the the improved Gerchberg-Saxton (GS) algorithm, and the required phase distribution of the diffractive optical elements is finally obtained. When compared with the traditional GS algorithm, the proposed algorithm can effectively suppress the speckle and ringing effect and obtain an output beam with improved uniformity.