Disadvantages of the coal seams in China are low permeability, low reservoir pressure and low gas saturation, which mean that the classical seepage model can't adapt the gas reservoir completely because of variability of the fluid seepage. The threshold pressure gradient exists during the gas flowing in low permeability media based on the experiment using the RMT-150B Rock Mechanics Testing and the coal samples from Sijiazhuang coal mine. For seepage model decrypting the characteristics of the fluid migration accurately in the low permeability seam to guide production practices, based on the migration law of the coalbed methane (CBM), continuity equation of fluid and Darcy's law, a three-dimensional, two phase flow seepage model considering threshold pressure gradient is presented to describe the characteristics of the fluid flow in low permeability seam. Finally the paper decrypts finite difference method to the model, which lays foundation for numerical simulation ofCBM reservoir.