Abstract: For typical strong and medium expansive soil and rock of the middle route of the South-to-North Water Diversion Project, mineral composition and its correlation with expansion and contraction indexes are studied through indoor soil tests and X-ray diffraction experiments. Qualitative analysis for microstructure is carried out by using the scanning electron microscopy images, and quantitative analysis for microstructures of pores and microstructural units is performed based on MATLAB. Microstructures such as particle morphology, structural type and microfracture distribution and their correlation with engineering characteristics are discussed. The results show that the content of clay minerals in expansive soil and rock is high. Strong expansive soil and rock have flocculation structure, flat aggregate and curly sheet-shape grains, and medium expansive soil and rock have turbulence and granular stack structure, flake aggregate and single grains. The former has higher void ratio, its microfracture with obvious orientation is well developed, and its pore fractal dimension and particle size are lower than those of the latter. The microstructural units of expansive soil and rock have low circular degree and high complexity. The orientation of expansion rock is significant and its particle fractal dimension is low. The fractal dimension has the same variation as the expansion potential and void ratio. The microcomposition, microstructure and physical properties exhibit linear and negative exponent correlations.