A floating Ocean Thermal Energy Conversion (OTEC) plant requires a large-diameter Cold Water Pipe (CWP) to be attached to a floating structure. For the design of the mooring system and the CWP, a coupled analysis of a floating body, mooring system and CWP should be employed due to the huge mass of the internal fluid in the CWP. The aim of this paper is to construct a simplified coupled response model to facilitate the preliminary stage of the design. The equations of equilibrium and motion are derived based on modeling as a two-dimensional floating body and an elastic pendulum. In order to verify the applicability for a practical design and limitation of the present model, a 100 MW ship-shaped platform, a spread mooring system and a CWP with an inner diameter of 12 m and a length of 800 m are configured. The results of the extreme analysis in the frequency domain with equivalent linearization of drag force by using the present model agree well with the time domain coupled analysis using OrcaFlex. Subsequently, the influence of the design parameters for CWP to the coupled responses is also clarified by a parametric study combining the bending stiffness, the linear density and the boundary conditions. The proposed model will facilitate the preliminary study with a large number of design trials, and a comprehension of the results of numerical simulations and model experiments.