Although satisfactory treatment effect was achieved from a recent foundation reinforcement practice, where a foundation composed of collapsible loess was treated with the vibratory probe compaction method in northwest China, the mechanisms of the reinforcement was unclear. To reveal the reinforcement mechanism, a series of experimental tests, including scanning electron microscopy, mercury intrusion porosimetry, and X-ray diffraction, were carried out on loess samples before and after the treatment. A total of 600 particles were identified, and the particle diameter, perimeter, sphericity, directionality as well as the pore distribution were analyzed from a quantitative perspective. Based on the microscale analysis, it is indicated that the propagation of vibration caused the destruction of cementation of soil particles in the initial state and, at the same time, the size of the particles decreased while the sphericity increased. The particles of the intact samples were found to be directional, whereas the directionality was minimized by the vibratory probe compaction process. The broken small particles gradually accumulated and filled the pores of the soil skeleton under gravity, which lowered the porosity and altered the pore size distribution.