Floating of the basic building blocks is the effective method to solve the problem that planetary gear train is uneven load, but it change the mode of planetary gear train. Firstly, it is necessary for this kind of gear train to research their inherent characteristics in design process. So the resonance region will be avoided. Therefore, vibration characteristics of planetary gear train in four conditions were researched, which included the disc planet carrier floating, the sun wheel floating, the sun wheel and disc of planet carrier floating and without floating. The three-dimensional modeling software CATIA was used to establish the parametric three-dimensional solid model of the planetary gear train. Then interference was analysed. Through the finite element software WORKBENCH, the modal analysis was carried on that of planetary gear train in four conditions, and respectively to extract the first ten steps natural frequency and vibration mode of this four conditions, and the results was analyzed and compared. The reference and basis was provided for avoiding resonance region of the basic building blocks floating planetary gear train in the design phase.