Based on the offshore environmental conditions in China, a new type of semi-submersible floating foundation is proposed to support 5 MW offshore wind turbine. Focusing on this new type of semi-submersible Floating Offshore Wind Turbine (FOWT) system, the aerodynamic loads on the rotor is calculated based on the blade element momentum theory. The hydrodynamic performance of the foundation is calculated based on the three dimensional potential theory. By adopting the bar model and beam model respectively, the dynamic responses of the flexible bodies are calculated based on the finite element method. The aero-hydro-structural coupling dynamic model is established. Considering the cut-in, rated and cut-off sea states, the dynamic responses of the FOWT is calculated in time domain. The results show that: the new-type of semi-submersible FOWT foundation is of good motion performance under different operational conditions, and the mooring tension meets the safety requirement; the blade tip deformations in the rotation plane are significantly larger than those out of the rotation plane, and the larger deformations are observed under the rated sea condition.