The invention discloses a support device for vertically installing a planetary gear transmission floating mechanism of a gearbox. The support device mainly comprises a primary planet carrier, a primary gear ring, a primary planet wheel, a primary planet shaft, a primary sun wheel, a secondary driving flange, a secondary gear ring, a secondary planet wheel, a secondary planet carrier, a secondary planet shaft, a secondary sun wheel and a bearing, wherein a spherical roller thrust bearing is arranged at the lower end of the primary sun wheel; a radial positioning cylindrical surface and an axial positioning thrust disk are designed on the end face of the secondary driving flange, and a floating mechanism support device in planetary gear transmission is constructed; the spherical roller thrust bearing can bear the radial load and axial load of a floating part, and can be used for automatically adjusting the centering error of the floating mechanism. The radial positioning cylindrical surface on the secondary driving flange can be used for radially positioning the floating part through the secondary gear ring and limiting the radial displacement of the floating part, and the axial positioning thrust disk is used for limiting the axial transmission of the floating mechanism.