The invention belongs to the field of structure engineering, material engineering and hydraulic and hydro-power engineering, and provides a fixing method for repairing an old concrete gravity dam with cracks through FRP. According to the method, the characteristics of low weight, high strength and corrosion resistance of FRP pieces are fully considered, an epoxy resin structure mixed with coupling reagents guarantees reliable bonding of the FRP pieces and the concrete gravity dam, and force is transmitted together. The grinding mode is used for embedding the FRP pieces with ribs to a non-congealed cement mortar face layer for repairing, and the FRP pieces and the non-congealed cement mortar face layer are tightly bonded and mechanically meshed. Rivets are used for mechanically anchoring the edges of the FRP pieces which is prone to stripping damage, the FRP pieces can not be easily stripped or subjected to brittle failure, and further expanding of the cracks is restrained. The method is simple and can be conveniently applied to engineering, the method can be used as an effective way for bonding, fixing and repairing the old concrete gravity dam with the cracks through the FRP pieces, design and construction for reinforcing the old concrete gravity dam through the FRP pieces can be guided, the expected protection requirement can be met through the design, and unnecessary economic waste is avoided.