Abstract: For the easily formed clogging phenomenon in the reinforcement process of vacuum preloading, the indoor tests under the conditions of different particle composition and different vacuum pressure value combined with the indoor vacuum preloading model test had been conducted. The formation features and mechanism of the siltation mud had been revealed, and the forecast model of drainage amount and the average moisture content had been built up preliminarily. The results show that the thickness of siltation mud is large with low moisture content and compactness due to the seepage paths are wide and long under lower vacuum degree, and the diameter distribution of pores is wide and uniform. However, the seepage paths are narrow and short under higher vacuum degree, the thickness of siltation mud is small with high moisture content and compactness, and the diameter of pores is small and the distribution is uniform. The higher clay content, the smaller the pore diameter, the higher degree of the concentration, and the more stable of the filter layer structure.