Abstract: Assuming the unit weight of the filled silty slurry increases linearly, this paper presents the formulation of a geotextile bag and develops a numerical analysis method to study the mechanical behavior of large scale geotextile bags. This method is validated by a case study. It is found that the tension of the large scale bag is much greater than that of a small bag at the same pumping pressure. Hence, the large scale geotextile bag should be kept at low pumping pressures in order to satisfy the strength requirement. At such low pumping pressures, the cross-section of the geotextile bag is flat with horizontal size much longer than vertical size, which is different from the circular tube of a small scale geotexile bag.