Iron oxides is one of the soil active components and can adsorb external pollutants such as heavy metals in soil. Hematite (α-Fe₂O₃) is the most common and stable iron oxides in tropical and subtropical soils. Due to its various morphologies, small particle size, and relatively strong surface reactivity, hematite can act as a strong adsorbent for soluble contaminants in soils and sediments. Al-substitution in hematite is ubiquitous in nature and affect the microstructure of hematite and its adsorption properties of metal ions. Thus Al-substitution in hematite strongly affects the environmental behaviors of hematite. However, the microstructure caused by Al-substitution and its inner relationship with the surface reactivity of hematite remain unclear. In this study, the crystal structure of Al-substituted hematite was characterized by high-resolution electron transmission microscopy (HRTEM) and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Acid-base titration and Pb²⁺ adsorption experiments were performed to investigate the surface reactivity of Al-substituted hematite. HRTEM images revealed that the proportion of (001) facets on hematite increased from 44.0 ± 3.8% to 84.8 ± 6.5% with increasing Al-substitution. HAADF-STEM images indicated that Al-substitution resulted in more vacancies of Fe atoms on hematite (001) facets. At these Fe defect sites, additional singly (≡FeOH^-0.5) and triply (≡Fe₃O^-0.5) coordinated hydroxyl sites were formed. The weight loss due to dehydrogenation followed the order of H-10 (6.2%) > H-5 (3.3%) > H-0 (1.7%). Besides, the relative percentage of surface oxygen atoms in the surface hydroxyl sites of Al-substituted hematite was consistent with the results of thermogravimetric analysis. At pH 5, the relative charge density of H-0, H-5 and H-10 was 73, 94 and 241 mC/m², and their adsorption capacity for Pb²⁺ was 0.82, 1.23 and 3.51 μmol/m², respectively. A similar trend was observed at pH 6. The results of pH-edge experiments further revealed that Al-substitution strongly enhanced the adsorption affinity of hematite for Pb²⁺. These results indicate that the formation of more defect sites with increasing Al-substitution can significantly enhance the surface charge density and Pb²⁺ adsorption capacity of hematite. These results also clarify the relationship between defect structures and Pb²⁺ adsorption of Al-substituted hematite and expand the understanding of the surface reactivity of Al-substituted iron oxides for various heavy metals.