A caisson and a vertical circular cylinder floating on the surface of fluid compose a wave power device. The hydrodynamic fluid/solid interaction becomes more complex due to the caisson. In this paper, in order to investigate the effects of the caisson on the cylinder's hydrodynamic coefficients and exciting forces, the expressions for velocity potentials are derived in the presence of an incident linear wave by use of an eigenfunction expansion approach. A set of theoretical added mass, damping coefficient and exciting force expressions are obtained. Two approaches are proposed to calculate the exciting forces and they match with each other well. Finally, Analytical results of added masses, damping coefficients and exciting forces are given for the different sizes of the caisson.