By using modified Gerchberg-Saxton algorithm, a kind of phase diffractive element is designed for homogenizing the irradiance on the focal plane in inertial confinement fusion (ICF) experiments. Using a packed array of these Pure-phase elements in conjuction with a primary focusing lens, it can transform a large laser beam into an arbitrary shape and size focus spot with a uniform profile, sharp pattern edge and very small sidelobes.