The utility model discloses an off-shore steel platform reinforcing device adopting underwater grouting, and comprises a reinforcing rod piece, wherein two ends of the reinforcing rod piece are respectively fixedly connected with an open-close type collar clamp, each open-close type collar clamp comprises two ring-shaped steel plates and a straight axle steel pin for connecting the two ring-shaped steel plates, and the two ring-shaped steel plates are connected through threads; the internal diameter of each open-close type collar clamp is slightly larger than the external diameter of the rod piece to be reinforced; sealing rubber is arranged at the open-close part of the open-close type collar clamps and on the inner sides at the two axial ends of the collar clamps; a grouting hole and a get pulp hole are formed on each open-close type collar clamp; and the open-close type collar clamps are in grouting connection with a structure to be reinforced. The outer layer of the whole device is coated with a corrosion protection coating. In the utility model, the reinforcing device is connected with the rod piece to be reinforced through the underwater grouting manner, so that the hot work is avoided, the operating risk is reduced, the reinforcing effect is reliable, and the construction difficulty is low.