The marine environment is complicated, and only considering the wind load cannot reflect the mechanical characteristics of the offshore wind turbine well. Therefore, it is of great significance to study the mechanical characteristics of the offshore wind turbine tower under combined loads in the offshore wind power engineering. Based on the data of 6 MW offshore wind turbine in Qidong sea area in Jiangsu province, GH Bladed software is used to carry out numerical simulations on the mechanical characteristics of the offshore wind turbine tower under combined loads, and the main conclusions are as follows: (1) the peak shear force on the bottom of the wind turbine tower under combined loads caused by the wind and wave increases by 72% and the peak bending moment by 8.3% compared with that under the wind load. (2) The wave load has a significant effect on both the shear force and bending moment on the bottom of the wind turbine tower. When the relative wave height is 4, the relative shear force and the relative bending moment on the bottom of the wind turbine tower are 3.04 and 1.44, respectively. (3) The peak shear force on the bottom of the wind turbine tower under combined loads caused by the wind and earthquake increases by 76% and the peak bending moment by 21% compared with that under the wind load. The research results can provide theoretical support for the design and construction of offshore wind turbines.