The consumption of concrete has increased annually due to the development of water conservancy engineering; however, the mechanical properties of concrete are subjected to the pore structure within. Based on X-ray computed tomography (XCT), this study analyzed the influence of different iron ore tailings dosages on the pore structure from multiple perspectives, such as porosity, pore location distribution and size distribution. The grey relational analysis and regression analysis were used to categorize the pores into harmful pores and less harmful pores. The linear quantitative model was established to analyze the relationship between concrete strength and porosity of harmful, and less harmful pores. The results showed that the 3D porosity and average pore size gradually decreases with the increase of iron ore tailings dosage. More pores scatter in the edge area than in the center of the specimens, with pores of 150-250 μm distributing most widely under different dosages. In terms of compressive strength and splitting tensile strength, the surface area boundary for the harmful pores and less harmful pores are 70 and 30 mm2, respectively. The proposed quantitative strength-pore structure model integrates the effects of porosity and pore size distribution, it can produce excellent verification results and predict the changing trend of concrete strength.