In this paper, we propose a speckle model for the diffusing object, considering the statistical nature of speckle phenomena, and the object illuminated with coherent light, the receding process of a specklegram is discussed and the formulae of specklegram are obtained. The formulae indicate that the frequency content ralates not only to the transform function of the imaging system, but also to the original frequency content of the diffusing object. Conclusions are confirmed by experiments. Then the formulae oftthe diffraction halo have been Seduced, which agree with the experiments well and can explain the speckle structure in the halo.