Because of the characteristics of water swelling and drying shrinkage, the mechanical performance of expansive clays in semi-arid areas deteriorates with changes in humidity and temperature. Hence, investigating the moisture migration in expansive clays is of great significance. Herein, scanning electron microscopy and water migration tests of unsaturated soil were carried out to address the water variation law and microscopic mechanisms in unsaturated expansive clays. It was found that small- and medium-sized pores (2–10 m) are dominant, and these pores are conducive to an increase in the water content in the soil. Soil with a high water content exhibits a flocculent structure, characterized by a small fractal dimension and a high relative hydraulic conductivity. Conversely, soil with a low water content displays a dispersed structure, featuring a large fractal dimension and a low relative hydraulic conductivity. The water migration tests, which were carried out at constant temperatures (5, 20 and 40°C) and variable temperatures (15–25°C), show that the amount of migration at 5 and 20°C was about 25–40 and 40–60% of that at 40°C, respectively. The amount of water migration at a constant temperature was obviously lower than that recorded at variable temperatures. The research results have provided technical support for the effective control of soil change.