In wind turbine planetary gearboxes, the sun gears are generally designed to float in the radial direction to achieve the uniform load distribution among different planet gears, but will lead to gear mesh errors and cause complicated vibration signals on the contrary. The floating sun gear is misdiagnosed as a distributed defect easily. Additionally, the floating sun gear's rotating motion and excitation force are too disorderly to be characterized by mathematical or empirical formulas. In the paper, a set of rigid-flexible coupling dynamics models of planetary gear train are built to research vibration characteristics of the floating sun gear under different conditions. Depend on the model-based vibration analysis, the rotating motion of floating sun gear is revealed well, which can be characterized by its frequency features, meanwhile mechanisms of modulation sidebands indicating different kinds of defects are deduced clearly. The experimental data keep good consistency with the simulation signals, which verifies the validity of the proposed well. Vibration feature indicators are obtained to distinguish fault signals and diagnose the distributed or localized defect of floating sun gear effectively, which is helpful to avoid the misdiagnosis of floating sun gear. (C) 2019 Elsevier Ltd. All rights reserved.