Support structures for tidal current power (TCP) generators can be divided into self-weight embedment, fixed-pile, and mooring types. Mooring-type TCP systems use buoyancy and mooring lines to stay afloat. Mooring-type support structures can be installed in any depth of water, so the installation process is simple, and costs can be significantly reduced. However, compared to other types of support structures, the motion of mooring structures is relatively large and requires an optimal system design to maximize power generation and secure dynamic motion stability. In this study, an optimal design is suggested for the mooring system of a floating TCP system using OrcaFlex 10.1a, a time-domain analysis program. The mooring system was developed by considering the sea environment and meets the design criteria for a wave energy converter (WEC) and TCP. Its pitch motion and yaw motion were maintained within approximately 3o. The results of this study could be used as basic information for the application of underwater floating TCP systems in various sea conditions.