The invention provides a device for synchronously expanding a rotary body in a radial direction. The device comprises a central shaft, the rotary body, an expanding mechanism, a rotary frame and a positioning sleeve, wherein the central shaft is used for supporting a rotary part and transmitting motion, torque or bending moment by rotating along with the rotary part; the rotary body is used for rotating a to-be-processed part or a part which synchronously rotates along with the central shaft; the expanding mechanism is used for synchronously expanding the rotary body in the radial direction; the rotary frame is used for connecting the central shaft to the expanding mechanism and axially limiting the rotary body; and the positioning sleeve is arranged between the rotary frame and the central shaft, and is used for guaranteeing that the rotary frame and the central shaft have relatively high coaxiality. After the rotary body is arranged on an expanding mechanism, a bolt is expanded by screwing up and the expanding sleeve is driven to axially move, so that wedge blocks get close to or get away from the center; and the tapered surfaces of the wedge blocks and the tapered surface of the expanding sleeve are the same in taper and are in tight contact, so that automatic centering can be realized; and the radial motion distance of all wedge blocks is the same, and the effect of synchronously expanding the rotary body is finally achieved, so that the rotary body and the central shaft are guaranteed to have relatively high coaxiality.