A method for reinforcing an underwater pier of a bridge by using an FRP pipe achieves reinforcing the underwater pier of the bridge by installing a positioning piece, wrapping with a steel wire mesh,manufacturing a self-locking FRP pipe, winding the self-locking FRP pipe, fixing the connecting part of the self-locking FRP pipe, activating the bottom sealing strip, and pouring filling material, wherein the self-locking FRP pipe is fastened by the moving clamp of per se, and underwater adhesive is injected into the glue injection cavity of the moving clamp so that the self-locking FRP pipe is bonded into a whole body, and the filling material is injected into the gap formed between the self-locking FRP pipe and the pier to be reinforced so as to finish the reinforcing of the underwater pierof the bridge. According to the invention, the process is simple, the self-locking type FRP pipe is fastened at one time, pre-fastening by using a belt is not needed, the construction process is reduced, the anti-corrosion performance is good, a cofferdam or a formwork is not needed, the labor intensity of workers is reduced, the pier can be directly reinforced under the water, the influence on achannel is reduced, the method has wide applicability, the construction is quick and convenient.