To prevent the overturning of a caisson on a mound due to tsunami overflow, the effectiveness of the change in the configuration of its landward upper part and the installation of a mooring cable at its seaward side is investigated using a coupled fluid-structure interaction model. Numerical results show that the change to slopes milder than 1/2 whose landward edge is under the still water surface can prevent the overturning of the caisson, suggesting the enhancement of its tenacity. This effect can be assessed using the scale of vortices formed at the landward side of the caisson. Furthermore, the mooring cable would be an effective countermeasure to enhance the tenacity of the caisson. However, the strength of the mooring cable and its attachment to the caisson and the mound needs to be assessed because of the action of large impulsive force.