The low permeability of coal seams is a key factor restricting gas extraction. The multiscale pores in low-permeability coal make coal permeability present the multiscale characteristics. However, the conventional steady-state method cannot measure the multiscale permeability of low-permeability coal well. In this study, a unidirectional multiscale dynamic apparent diffusion model is proposed as an analytical model, and a multiscale dynamic apparent diffusion coefficient is defined. In addition, an experimental method for measuring low permeability from macroscale to microscale pores is provided. The experiments of gas desorption flow in the unidirectional, radial, and spherical directions were conducted to compare with each other. The research results show that (1) the apparent diffusion coefficient and apparent permeability decrease with time because of the multiscale pore structure in coal. (2) The multiscale dynamic apparent diffusion model can accurately describe the full-time process of the unidirectional gas desorption flow in coal. (3) The proposed model shows a broader applicability with a comparison to the current models.