Pore structure is one of the most fundamental parameters in reservoir characterization and is closely related to the calculation/interpretation of absolute permeability and flow capacity. This study aims to characterize the pore structure and evaluate its effect on porosity and permeability for unconventional reservoirs. We proposed an analytic expression of absolute permeability, considering the threshold pressure gradient by combining the capillary seepage model and fractal theory. The absolute permeability is described as a function of reservoir porosity, fractal dimension of the pore throat and capillary tube, and the maximum pore throat radius. We further conducted rate-controlled mercury injection experiments to determine the pore-structure characteristics of samples from the Chang-7 formation, located in the Yanchang area. These samples are characterized by an average pore radius ranging from 115.79 to 129.07 mu m and an average throat radius ranging from 0.15 to 0.41 mu m. The experimental results suggest that the absolute permeability is highly related to the pore-throat ratio. Calculation and comparison demonstrate a relatively high accuracy of the proposed expression in absolute permeability estimation. Additionally, sensitivity discussion indicates that absolute permeability positively correlates with porosity, average pore radius, average throat radius, mainstream throat radius, and maximum connected throat radius.