Recently, there have been many researches to find the method for minimizing the damage from natural disasters. The disasters in the coast such as typhoon and tsunami are being studied to reduce them, and one of possible methods is to locate some structures for the protection of wind and wave. The most eco-friendly method minimizing the destruction of ecological system is to construct the forest of damage prevention, which has been actively investigated for the last decade. However, we know the fact that the coastal forests can reduce the damage of natural disaster, but the mechanism how they work inside the forest and in the flow field of wake is not yet precisely understood, so there has been no quantitative investigation on the damage prevention effect. The real tree is not rigid but flexible, which means that trees are oscillated with the environmental wind, so this effect is not sufficiently considered in the past researches. In this study, applying FSI(fluid-structure interaction) for a single tree model, the effect of energy dissipation is predicted in the wind field around a forest.