The invention discloses an insulating support structure in an inflatable impulse voltage generator. The structure comprises drum-type supporting keys (1) and a ball bearing structure (2). The supporting keys are placed around a voltage sharing ring (4), and accordingly enough mechanical strength and stability are guaranteed. The supporting keys (1) are connected with a tank body (3) through the ball bearing structure so that the insulating support structure can move forward and backward so as to be conveniently assembled and disassembled. The supporting keys (1), the ball bearing (2) and the voltage sharing ring (4) are connected through rigid bolts to guarantee supporting strength of the insulating support structure. A column body of the drum-type supporting keys is of a drum shape so that normal component of the field strength of the surfaces of the supporting keys can be reduced, an internal electrode is of a special structure, electric field distribution can be more uniform, and the insulating support structure can bear voltage higher than nominal voltage of the insulating support structure. All connection portions of the supporting keys are of a fillet structure, and electric field distribution is improved.