The evolution of nanoscale microstructure of expansive clay during drying-wetting cycling significantly affects their macroscopic hydro-mechanical properties. Due to limitations in testing scope and accuracy， the existing test methods are difficult to quantitatively characterize the nanoscale microstructural evolution of expansive clay. In this paper， the first attempt is made to quantitatively characterize the nanoscale microstructural evolution of expansive clay during drying-wetting cycling using small-angle neutron scattering technique. The results show that the microstructural pore size distribution of expansive clay exhibits bimodal characteristics in the range of 2~100 nm， corresponding to inter-sub-stack pores and inter-particle pores， respectively. For the whole drying-wetting cycling， the overall volumetric strain of these two types of microstructures increases by about 11% and decreases by about 10% during drying and wetting process. Among them， the inter-sub-stack pore variations dominate the microstructure volume changes in this detection range. Comparative analysis of the neutron scattering test results and the isothermal adsorption physical model shows that the potential physical inducement of the volume deformation inter-sub-stack pores is external adsorption. This study reveals the microscopic mechanisms by which drying-wetting cycling affects the hydraulic conductivity， swelling， and shear strength of soils， providing a theoretical basis for the regulation and improvement of expansive soil microstructure.