Extensive assessment about the efficiency and quality of the power production is important to the floating wind turbine (FWT) development. The power performance of a wind turbine depends on turbine dynamics, control strategy, and atmospheric conditions. As for FWTs, the additional floater oscillations should be particularly incorporated. In this paper, extensive simulations are conducted for a Spar‐type FWT using the FAST software. The relative wind velocity introduced by floater oscillations and the corresponding power capture mechanisms are analyzed in detail. A concept of equivalent turbulence intensity is proposed to generally describe the intensity of floater oscillation. It is observed to satisfyingly serve as an independent variable for FWT power production. The mapping relationship between the FWT power generation and the equivalent turbulence intensity is constructed. Results show that the FWT output power increases linearly with the equivalent turbulence intensity square in the below‐rated region, while it is well regulated around the rated power by the bladepitch controller in the above‐rated region. The normalized power fluctuation augments linearly with equivalent turbulence intensity over the whole operating wind speeds except the rated‐nearby region. The findings are potentially helpful for the power forecasting, controller development, economical evaluation, and wind farm optimizations.