With the increasing application of digital hologram, how to acquire high quality of reconstructed image of digital hologram has been a very important research topic. Speckle noise has been one of the most difficult problems in the coherent imaging system because of its poor statistical regularity. Based on the statistical characteristics and the formation cause of speckle noise in the reconstructed image of digital hologram, the way to reduce speckle noise is put forward. The frequency spectrum of reconstruction with lower energy is extracted from the reconstructed image of hologram and considered as approximate frequency spectrum of speckle noises. Then it is replaced with random data and the reconstructed image with different independent speckle noise is obtained. Lastly speckle energy is gradually reduced when the independent reproduced speckle noises are iteratively superimposed. As a result, the speckle noise can be reduced greatly and the quality of reconstructed image can be improved. The validity of this method is proved in theory and experiment. It offers a entirely new thinking and practical way to reduce the speckle noise in the reconstructed image of digital hologram.