Objective: Floating offshore wind turbines (FOWTs) experience irregular motion because of their unique structure under severe deep-sea conditions. The floating motions of the FOWTs exert specific effects on their aerodynamic characteristics, manifested in the periodic variation of the blade angle of attack, instability of aerodynamic loads, and fluctuations in power output. This study investigates the 6 degrees of freedom (DOF) motion of the FOWT, simplifies the complex floating motion to harmonic motion, and quantitatively analyzes the influence of different factors, such as amplitude and frequency on the aerodynamic characteristics. The power generation of the FOWT under the floating motions is determined, and the total energy output is calculated based on this. Methods: By utilizing the integrated analysis software OpenFAST, the aerodynamic characteristics of the FOWT under harmonic motion are investigated in this study. The blade element momentum theory is employed to compute the aerodynamic loads acting on the blades in OpenFAST. Meanwhile, the structural response of the FOWT is determined based on the structural dynamics. The coupled calculations are conducted based on Kane's method in multibody dynamics. The mass, stiffness, and damping matrices of the platform mounted on the wind turbine are defined within the ExtPtfm module of OpenFAST to establish a superelement model. Following the principles of structural dynamics, a time-history harmonic load is applied to the model to induce the harmonic motion of the FOWT. This study focuses on the NREL 5-MW baseline wind turbine as the subject of investigation. The thrust and torque coefficients of the turbine during harmonic motion are statistically analyzed. In addition, the axial aerodynamic loads and aerodynamic torques experienced by the FOWT under both surge and pitch motions are examined through time and frequency domain analyses, followed by a comparison of power output and energy generation between fixed and floating turbines. Results: The statistical analysis of the thrust and torque coefficients reveals that, for the surge motion with an amplitude of 5 m and a frequency of 0.1 Hz, the coefficients of variation for the thrust coefficient is 8 times greater than that of a fixed wind turbine, whereas the torque coefficient increases by a factor of 23. For the pitch motion with the same frequency and an amplitude of 5°, the thrust and torque coefficients increase by factors of 20 and 44, respectively. Time and frequency domain analyses of the aerodynamic loads acting on the wind turbine (both axial aerodynamic load and aerodynamic torque) indicate significant fluctuations in the aerodynamic loads under both surge and pitch motions, with additional components induced by the motion. Furthermore, the power generated by the wind turbine exhibits considerable fluctuations because of the effects of both surge and pitch motions. Conclusions: In the case of the wind direction being perpendicular to the rotor plane, both surge and pitch motions in all 6 DOFs have a more significant effect on the aerodynamic characteristics of the FOWT. Both surge and pitch motions cause periodic variations in the aerodynamic loads, with the amplitude and frequency of the motion influencing the nature of these fluctuations. Moreover, both surge and pitch motions lead to instability in power generation. However, the total energy produced over time remains largely unaffected.