Abstract: The critical suction is the key control factor for the installation of suction caissons in sand. The existing methods mainly rely on seepage analysis to determine the critical suction, which does not consider soil deformation and consequently might not realistically reflect soil failure under the critical suction. By using the stress-seepage coupled finite element method, the critical suction is determined based on soil displacements inside a caisson. After examined against the field tests, the model is utilized to study the differences between the critical suctions determined by stress-seepage coupled and seepage analyses, and the failure mode of soils under the critical suction is discussed. The main conclusions include: (1) The vertical displacements of the soils inside the caisson exhibit accelerating growth under critical suction, reflected by an inflection point in the soil displacement-suction curve. (2) When the embedment ratio is relatively large, the seepage-based methods might underestimate the critical suction. (3) The soils inside the caisson show wedge-shaped failure mode and uniform upheaval failure mode under the critical suction when the embedment ratio is relatively small and large, respectively.