Abstract: During the installation of suction caissons, there exists the phenomenon of soil plug inside the anchor, which will hinder the further penetration of the anchor. This phenomenon hinders the installation to reach the predetermined depth, resulting in a great reduction in bearing capacity. In this study, a series of tests under 1g are carried out to analyze the formation mechanism of soil plug. The negative pressure installation of an anchor model is conducted in sand, and the whole process is recorded by a high-resolution camera. The pore pressure transducers are used to measure the internal and external pore pressures. During the penetration, the fine sand particles move upwards and reach the top surface due to upward seepage flow within the sand inside the anchor. With the increase of the penetration depth, the height of the soil plug increases progressively. It is found that the soil plug is caused by the swelling of the soil inside the suction anchor, and the permeability coefficient increases due to the swelling of the inside sand. The variation of the permeability coefficient in the process of penetration is investigated, and it is used to improve the Houlsby & Byrne’s model. The modified model can accurately predict the suction in the penetration process and provide a theoretical basis for the design and construction of suction caissons.