Abstract: The suction caisson foundation is a key foundation form in offshore engineering, with the suction-assisted penetration offering low-noise and low-environmental-impact installation. The accurate determination of the critical suction is essential to prevent the excessive deformation of soils and formation of soil plugs in sand. However, the studies on suction-induced soil deformation during caisson installation are limited. The physical model tests and deformation-seepage coupled numerical simulations are conducted to analyze the relationships between soil displacement seepage response and suction and failure modes under varying embedment ratios. A criterion for determining the critical suction based on soil deformation is proposed. The results indicate that: (1) As the suction increases, the vertical displacement of the soils begins to increase rapidly, accompanied by a sudden increase in the permeability of the soils. The above abrupt increase in soil displacement can be used as a criterion for determining critical suction. (2) The critical suction increases significantly with the increase in the embedment depth over diameter ratio of foundation. (3) For smaller embedment ratios, the soils inside the caisson exhibit a wedge-shaped failure under the critical suction, while for larger embedment ratios, they exhibit a global upward heave failure mode.