An improved Gerchberg-Saxton (G-S) algorithm is proposed to realize the accurate design of the diffractive optical elements based on annular beams. This algorithm ensures a small sampling interval on the output plane and suppresses speckle. Compared with the design results of the conventional improved G-S algorithm without speckle suppression, a uniform spot with better performance is obtained by the proposed algorithm. The simulated results are in agreement with the experimental results.