Abstract: The lumped mass method was adopted to build the mathematical model of falling-net, and the untrammeled settlement process was studied by numerical simulation method. The simulated results which were verified by measured data in settlement test of four different weights show that the change of falling-net space shaped in the untrammeled settlement process. The study shows that:1) the shape of net mouth became approximate circular along with the settlement, obtuse on the side of against the current, and salient along the direction of the current on the other side; 2) there was significant difference in tension, hydrodynamic force and sinking speed of the mass point in the net mouth among four weights and all of them increased with the weight; 3) the tension of the mass point in the net mouth increased sharply at first, then showed a decrease-increase fluctuation, and increased slowly at last. The hydrodynamic force and sinking speed decreasd sharply at first, then showed an increase-decrease fluctuation, and decreased slowly at last; 4) the study suggests that the sinking speed continues to increase with weight in the allowable weight range of falling-net.