The mooring system keeps the position of a floating body such as a ship or offshore structure under environmental loads. To analyze accurately the motion of the floating body, the dynamic motion of the mooring system should be considered together. Simply, the mooring system can be modeled as linear springs and then the analytic solution can be used for quasi-static analysis. For dynamic analysis, the FEM (Finite Element Method) is mostly used for the mooring system. However, existing researches using the FEM for the mooring system have been applied for a single floating body, not a multibody system such as a floating crane. In this study, the dynamic motion of the mooring system connected to the multibody system is analyzed using flexible multibody dynamics. Furthermore, the interaction between the mooring system and the seabed is considered using non-interpenetration constraints. Finally, the mooring analysis during the lifting operation of the floating crane is performed. As a result, it can be seen that this approach can be used for the accurate mooring analysis of the multibody system.