In this paper, the authors present a possible model for a new type of damping structure (abbreviated to FPL) composed of a flexible porous material such as polyurethane foam and a liquid such as water. The model can be used to design the FPL so that it is suitable for various target vibrating objects. In a previous paper, the authors introduced the FPL and presented the damping effect of the FPL on the vibration of two types of specimens: a cantilever iron plate with a weight of 5.2 kg and a rail piece with a length of 1.2 m and a weight of 72 kg. Good damping performance of the FPL was shown in the paper. The FPL costs less than other viscoelastic materials, such as Sorbothane, and is anticipated to have a better damping effect. In addition, it was shown that the damping effect of the FPL depends on the properties of the liquid and porous material as well as the vibration characteristics of the target objects. Consequently, it is important to make a simple model of the FPL widely available for its design and engineering simulation. In this paper, the authors derive and propose a possible model of the FPL based on a simple dynamic and macroscopic perspective.