Diffractive optical elements are designed to implement the wavefront transformation from a two-dimensional circular-Gaussian beam to a two-dimensional flat-top square beam and a two-dimensional flat-top triangular beam respectively with the weighted serial iterative (WSI) algorithm, which realizes the transformation of beam-shape and the uniform distribution of the amplitude. The computed results indicate that the energy transformation efficiency is up to 95.3%, 93.6%, arid the output amplitude satisfies the need.