The natural frequency of the semi wind turbine is a key parameter that influences whether the wind turbine can reach full load. However, the calculation of the natural frequency by using the commercial software for wind turbine design is insufficient in the accurate consideration of the stiffness, mass distribution, underwater added mass, hydrodynamic damping and mooring stiffness of the floating foundation. Then, the structure model of a four-column semi-submersible offshore wind turbine has been established for accurate simulation of the stiffness and mass distribution of the floating body and the tower. According to the low-order modal analysis results of the tower, the tower is simplified as a beam unit with weak spring boundary constraint. The first 20-order natural frequencies of the wind turbine are calculated, staring from the 7th order as the natural frequency of the structure. On the basis, the upper limit of the stiffness for weak spring boundary constraints is determined, and the effects of the added mass of the semi-submersible foundation, hydrodynamic viscous damping, structural damping and mooring stiffness on the natural frequency of the wind turbine are analyzed to propose a method to calculate and distribute the added mass. The natural frequency of the wind turbine can be increased by adjusting the diameter and the thickness transition of the middle section of the tower in the later stage of the design based on the low-order vibration mode of the whole wind turbine.