During the vacuum preloading of hydraulic fill ultra-soft soil, "soil pile" is often formed around drainage plate, which hinders the consolidation of the surrounding soil and even leads to the failure of the foundation treatment, but the forming mechanism of "soil pile" is still unclear. To this end, a model test was carried out to implement non-intrusive measurement of the surrounding soil deformation during vacuum preloading using the transparent soil experimental technique and the PIV (particle image velocimetry) technique. The research showed that the surrounding soil deformation of drainage plate was related to drainage rate. During the quick and steady drainage period, the surrounding soil of the drainage plate had obvious horizontal displacement, and the range of the horizontal displacement expanded gradually with the increase of the vacuuming time. During the slow drainage period, the vertical displacement was obvious in the surrounding soil of the drainage plate, and there was almost no horizontal displacement. The formations of "soil pile" and "soft zone" were mainly due to the transportation and gathering of the soil particles around the drainage plate. Compared with the deep soil layer, the shallow soil layer had finer particle size and lower self-weight, which was easier to transport under the seepage force, thus leading to the decrease of the radius of the "soil pile" along the depth, and the maximum radius of the "soil pile" could reach 11 cm. The density of the "soil pile" was higher than that of the "soft zones", and the compression of the "soil pile" was smaller under self-weight and "vacuum load", resulting in the formation of the "pile head" on the foundation surface.