Abstract:  With the water depth of offshore wind farm and the power of wind turbine increasing gradually, the traditional fixed foundations are not applicable. The floating foundations with excellent characteristics have become a research focus. The semi-submersible foundation and mooring system are designed for a 600 kW wind turbine with a horizontal shaft, and the water depth is 60 m. SESAM software is used to establish the finite element model of this semi-submersible platform. The motion responses are calculated in both frequency domain and time domain. Motion performance and mooring line tension are also investigated. This research has some guiding values for the development of offshore wind farm with floating foundation.