Second-order components of the wave loads on floating wind turbines may induce severe resonance at the natural frequencies of the structures and result in fatigue damage. A conceptual semi-submersible platform for the DTU 10 MW Wind Turbine was introduced and simulated by FAST to obtain the second-order responses under steady winds with collinear random waves. The dynamic responses were calculated according to three models, labeled as "1st", "1st + the full Quadratic Transfer Function (QTF)", and "1st + Newman approximation". The average and wave-frequency responses are similar based on three models. With a larger natural frequency, the pitch resonant responses differed more significantly between the Newman model and the full QTF model, as compared to the surge resonant responses. Thus, the method using Newman approximation is not accurate for second-order calculation. A high-frequency response appeared in the tower-top shear force spectrum only when the full QTF was utilized. Sensitivity studies were conducted with various wind speeds, significant wave heights, wave periods, and wave directions. The wind load significantly affected the pitch motion and tower-top shear force and functioned as aerodynamic damping in low-frequency and high-frequency ranges. The second-order responses exhibited quadratic relationships with the significant wave height. (C) 2020 Elsevier Ltd. All rights reserved.