To understand the time-variant characteristics of force and deformation of large-size geosynthetic tubes filled with slurry and to provide a calculation basis for tube design, the tension, shape, and height of a geosynthetic tube during the filling process were calculated using the dimensionless method. Two working conditions during the filling process were considered: one was the free filling condition, under which the pressure at the top of the geosynthetic tube was zero, and the other was the pump filling condition, under which the pressure at the top of the geosynthetic tube was greater than zero. It was found that, when a geosynthetic tube with a perimeter of L was filled with slurry with a density of ρ, the critical filling height from the free filling condition to the pump filling condition is 0.1L, the filling volume per unit length of the geosynthetic tube is 0.04L2, and the corresponding tension of the geosynthetic tube is 0.0025ρgL2. After the critical filling height was reached, a continuous increase in the pump pressure was needed to maintain the filling process. Both the required pump pressure and the tension of the geosynthetic tube increase nonlinearly and rapidly with the filling height.