A design method of diffractive element that can effectively suppress speckle noise is proposed. Based on the traditional Gerchberg-Saxton algorithm, the uniformity of the light spot is effectively improved without reducing the diffraction efficiency by selecting a special initial phase. Taking the Gaussian beam shaped into an annular flat-top beam as an example, numerical simulation and optical experiments are carried out. The results show that the speckle contrasts of the annular flat-top beams obtained by using the proposed method and the traditional Gerchberg-Saxton algorithm are 11% and 34%, respectively, which verifies the proposed method can obtain a annular flat-top beam with low speckle noise.