The aim of this study is to present numerical procedures for hydrodynamic analysis of a heaving-buoy-type WEC connected to a fixed jacket structure that is currently being actively studied. In order to accurately assess the effect of the float on the fixed structure, the augmented formulation procedure used in multibody dynamics was used to calculate the reaction force due to the float motion acting on the hinge point of the structure. The hydrodynamic coefficients and wave excitation forces acting on the floats were calculated using the three-dimensional frequency domain solver WAMIT. The wave radiation forces produced by the movement of floats were calculated using the Cummins’ equation in the time domain. Compared to the classical method of formula-derivation technique, the augmented formulation, which is an automated formulation process, has an advantage that it can be easily calculated even if the degree of freedom is increased. Thus, it can be easily extended to analyze hydrodynamic performance of various multibody ocean structures. To conduct the time integration, the implicit Runge-Kutta 4th-order method was applied to the classical method. A two-loop numerical integration with the Newmark-beta explicit method was used in the augmented formulation method. The responses of the float and reaction force acting on the hinge point were evaluated using the two numerical procedures. Finally, the calculation results of two floats for a multi-buoy HPA WEC platform were analyzed to emphasize the importance of reaction force analysis.