In this work, we used the channel multipoles, eigenfunction expansion and separation of variables method to determine the diffracted potential of the wave for a floating solid cylinder in the free surface of the water over a coaxial bottom-mounted cylindrical barrier in a channel. Also, the mathematical formulation of the proposed model based on the linearized water wave theory. The forces due to diffraction acting on the floating cylinder are described here based on the Bernoulli's equation of fluid flow. The effect of various parameters of that composite structure on the exciting force and overturning moment of the floating cylinder is investigated. The result shows that a significant influence of forces acting on the floating cylinder in the presence of a bottom-mounted barrier in the channel which gives an accuracy of the analytical results.