A diffractive optical element (DOE) with 8 levels is designed to implement the wavefront transformation with two-dimensional weighed serial iterative algorithm. The DOE converts a circular-Gaussian laser beam into a 10 μm×10 μm flat-top square focused spot, which realizes the transformation of beam-shape and the uniform distribution of the amplitude at the same time. The DOE is applied to high-density holographic storage, and it meets the needs of the uniform distribution of the incident focused spot. The simulated results show that the energy transformation efficiency is up to 91.2%, the uniformity error is 4.6%, and the error is below 0.023%, which nearly satisfy the need. The tolerance of the incident beam and the focus is analyzed too. Three sets of masks on 8 levels phase DOEs have been processed.