A braceless semi-submersible offshore wind turbine foundation is designed to solve the problem of poor heave performance. The hydrodynamic parameters, amplitude-frequency characteristics are analyzed considering the viscous damping. Analysis time domain motion characteristics and mooring tension under the combined wind and wave conditions. The results show that the heave performance of the new semi-submersible foundation is greatly improved. In frequency analysis, the wave incidence angle has little effect on heave. In the time domain analysis, it is found that the heave motion is mainly affected by the wave load. The wind load determines the average balance position of the surge and pitch, and the wave load determines the amplitude of the motion response, leading to a stronger fluctuation of the motion. Similarly, the average mooring tension is mainly affected by wind load. Under the action of wave load, the mooring tension amplitude increases, and the maximum mooring tension is determined by the combined action of wave load and wind load. The combination of rated wind speed, extreme wind speed and extreme sea conditions shall be considered.