The No. 2 gas field in the X depression is a low-permeability tight reservoir with a complex lithology, pore structure, and strong physical heterogeneity, and the conventional core porosity−permeability regression method does not meet the requirements of fine evaluation in terms of the accuracy of permeability calculation. The flow unit method has great advantages in improving the accuracy of permeability calculation, but the FZI calculation method is too ideal and weakens the influence of the pore structure’s heterogeneity, and it needs to be verified that the FZI in the study area has a good correlation with the pore structure before it can be used. Therefore, based on analyzing the permeability control factors of low-permeability tight reservoirs, we analyze the correlation between three pore structure characterization parameters and the flow unit index FZI, which proves that the flow unit index FZI in this area can characterize the permeability difference within different flow units. Based on FZI theory and the cumulative frequency division method, we establish a fine evaluation model of four types of reservoirs in the study area. Through the response characteristics and correlation analysis of the conventional logging curves, we select three combined curves, establish a multi-parameter equation, and apply it to the permeability evaluation of the cored section that is not involved in modeling. The application results show that the calculated permeability is in good agreement with the core analysis results, which provides a theoretical basis for the fine evaluation of low-permeability tight reservoirs.