Abstract: Sandbag cofferdam applies flexible load on soil foundation. The failure mode and sliding surface of soft soil foundation under sandbag cofferdam should be significantly different from that under rigid base. Aimed to get better understanding about this problem, a model test is conducted to simulate the construction process of sandbag cofferdam, in which the displacement of soil is observed. Based on the results of model tests, a new failure zone is proposed. The finite element analysis is used to verify the reasonability of this new failure zones.