The structure of fractures and pores has a dominant impact on the heat transfer-seepage-deformation process of a coal seam. Previous models have primarily used the cubic permeability model to characterize coal seam permeability properties. In this study, we developed a new multi-field coupling model, which includes fracture and pore structure, coal seam temperature, effective stress and gas seepage. Two major extraction scenarios were simulated: the unconstrained plane strain state and the uniaxial plane strain state. In addition, two microstructural parameters were applied to characterize coal permeability: the maximum fracture length and the fractal dimension for the fracture. The results show that the fractal seepage model provides a more realistic and reliable characterization of resource migration and extraction processes in unconventional reservoirs than the cubic-law permeability model. Compared with the cubic-law permeability model, the permeability calculated by the model proposed in this paper changes about 17.09%-91.56%. Furthermore, coal seam permeability is proportional to the maximum fracture length and the fractal dimension for the fracture. The permeability changes about 17.09% and 17.18% with the different fractal dimension, and about 87.17% and 91.56% with the different maximum fracture length. However, the fractal dimension and coal seam permeability are inversely proportional to seam temperature.