Abstract: In oxidative conditions, sulfides in black shale readily oxidize into expansive sulfate minerals, causing geotechnical issues such as subgrade deformation and slope failure. Scanning electron microscopy (SEM) and low-temperature nitrogen adsorption analysis were employed to examine the variations in swelling rate and microstructural characteristics of black shale under ferric ion oxidation. The results showed that as the FeCl₃ solution concentration increased, the black shale surface became more compact and partially flatter after the reaction. The pore adsorption capacity declined, hysteresis loops decreased, and the fractal dimension D₂ of pore structures was higher than D₁, indicating a more complex internal pore structure than the surface. During the reaction between black shale and ferric ions, gypsum was generated and filled some pores, reducing both pore volume and specific surface area, and lowering pore roughness and complexity. In swelling tests, samples with low initial water content had strong water absorption and a higher swelling rate. Under the condition of a 0.05 mol·L^-1 FeCl₃ solution, the swelling of black shale resulting from clay mineral hydration and the electrical double-layer effect could be effectively inhibited. Upon increasing the ferric ion concentration, the accelerated oxidation of pyrite generated expansive sulfates, causing a significant rise in the swelling rate. In a 0.2 mol·L^-1 FeCl₃ solution, the swelling rate reached 9.35%, marking a 147% increase in volumetric swelling compared to distilled water.