In this paper, a partially developed speckle model for millimeter-wave holographic imaging with specular reflection is established. The partially developed speckle model is combined with the convolution process of millimeter-wave holographic imaging. The number of equivalent scattering centers is deduced, and the relationship among speckle contrast, root-mean-square height, correlation length, and imaging resolution of a Gaussian rough surface is established. The Monte-Carlo simulation experiment and the speckle contrast analysis are conducted on the millimeter holographic speckle patterns of a random rough surface based on the near-field physical optics method. The results show that when the number of equivalent scattering centers is small, the speckle contrast estimated using the proposed model is consistent with those estimated using the stochastic integral method and the simulation experiment, which is superior to the estimation results of the conventional models.