In order to improve the uniformity of intensity distribution of the reconstructed images and improve the imaging quality in the design of the diffractive optical elements, the design algorithm is optimized by adding a mixed amplitude degree of freedom to the target image due to the fact that there must be leakage in the light field propagation process, and the effectiveness of the method was verified by designing a Fourier transform type diffractive optical element. The simulation results show that, compared without amplitude degree of freedom, the reconstructed image’s intensity uniformity of the elements designed by this method is reduced from about 14% to less than 1%, and the mean square error is reduced from about 10% to less than 0.1%, and the method is applicable to both Gerchberg & Saxton(GS) algorithm and simulated annealing algorithm.