Downstream sinking technology of flexible mattress has wide application in erosion protection、berth firm and bank protection. During sinking of flexible mattress, the distance get gradually between the landing position of the mattress and the laying mattress ship, then the suspense length and mattress’s self-weight are also increase because of the ship’s move. Therefore the tension in mattress is also change continuously, so the calculation of the tension becomes difficult. By using lumped mass method, the mattress can be separated into several lumped mass which are connected using non-mass springs, with the laying distance increase, the length and self-weight of the mattress can be increase gradually, and according to forces equilibrium equation, the tension and profile of the mattress can be obtained.