Second-order components of wave loads acting on the floating foundation for wind turbines may induce severe resonance and lead to fatigue damage at natural frequencies of structures. In this study, the INNWIND.EU Triple-Spar and the DTU 10 MW Reference Wind Turbine were simulated by utilizing software FAST to obtain the second-order responses of the floating wind turbine under selected steady winds with collinear random waves. Low-frequency responses at surge and pitch natural frequencies dominated the response spectra, which were underestimated by the first-order numerical model. A response peak appeared in tower-top motion spectrum in vicinity of the first-order fore-aft vibration frequency of the tower when the sum-frequency wave effects were considered. The second-order high-frequency responses arose when the full QTF was utilized, compared to results with Newman approximation. Different operating conditions with varying wind speeds, wave periods, significant wave heights and wave directions were selected to conduct the sensitivity study of the second-order responses.