Because of paucity of land, photovoltaic systems are increasingly being installed in coastal waters. However, specifications, such as the type and size of the structure applied to the offshore floating photovoltaic system, differ for onshore systems. This difference in specification results in numerous safety problems. Therefore, sea environmental conditions should be considered for the safety design of the floating body structure for photovoltaic systems. Accurate wave load evaluation and appropriate mooring line design for the target sea area are critical for effective design. In this study, a hydrodynamic design of an offshore photovoltaic floating structure was developed with glass-fiber-reinforced plastic (GFRP) as a structural member and appropriate arrangement of mooring lines. Numerical analysis of in-house code developed using KRISO was performed to optimize floating body motion stability and mooring line arrangement for an offshore floating photovoltaic power generation system with GFRP structural members. The results of the analysis confirmed the stability of the floating system and damage limit of the mooring line.