The utility model discloses a reinforcing device for a large formwork of an oblique-section concrete member. The reinforcing device comprises the large formwork, a counter screw, a gasket, a fastening nut, a wedge-shaped cushion block and a clamping base, wherein the wedge-shaped cushion block is arranged between the horizontal back edge of the large formwork and the gasket. The wedge-shaped cushion block has the same inclination angle as the horizontal back edge of the large formwork, and is attached to the horizontal back edge of the large formwork and provided with an open hole corresponding to an open hole in the large formwork. The open hole of the wedge-shaped cushion block is an oblong hole and is used for installing the counter screw to reinforce the large formwork. The clamping base is welded and fixed to the horizontal back edge of the large formwork, and the upper plane of the clamping base makes contact with the lower plane of the wedge-shaped cushion block. The wedge-shaped cushion block can be manufactured through waste steel plates on site and can be circulated. The clamping base is welded to the horizontal back edge of the large formwork and can limit the movement of the wedge-shaped cushion block, and therefore the better fastening effect can be achieved. The wedge-shaped cushion block is provided with the oblong hole so that the fastening effect of the reinforcing device can be guaranteed.