In wind turbine,there exists a so- called double- elastic support,i.e.,the gearbox and the generator arerespectively linked to the base with certain stiffness by leaf springs and elastic rubbers.Due to this special charac-teristic,there exists a natural axial misalignment between the output shaft of gearbox and the shaft of generator,which makes vibration signals of normal gearbox blend with serious high- order gear mesh frequency and smoothmodulation.In this paper,based on the detailed analysis of more than 1000 sets of field data,the vibration charac-teristics of gearbox,the main bearing,the generator bearing and the supporting equipment in wind turbine are dis-covered,and the influences of double- elastic support on the frequency characteristic of gearbox is verified.Moreo-ver,according to the structural characteristics of wind turbine and the analysis results of test data,the difficulties offault diagnosis in the transmission system of wind turbine and some possible solutions are analyzed.