The invention discloses an optical fiber grating strain sensor with high sensitivity, and belongs to optical fiber sensor technologies. The optical fiber grating strain sensor comprises bottom-layer sheet metal, middle-layer sheet metal and top-layer sheet metal which are detachably connected with one another. A strain measuring grating is mounted on the top-layer sheet metal, and the bottom-layersheet metal can be fixed onto a to-be-measured object; lever mechanisms are arranged in the top-layer sheet metal and the middle-layer sheet metal, and deformation of the to-be-measured object can betransmitted via the cascaded lever mechanisms between layers and can be amplified on the strain measuring grating. The optical fiber grating strain sensor has the advantages that the optical fiber grating strain sensor is of a three-layer sheet metal cascade structure, strain of the to-be-measured object can be amplified by multiple stages by the aid of the lever mechanisms, 'sensitivity enhancement type' amplification and 'centralization type' amplification theories are adopted for amplification structures, accordingly, amplification factors can be obviously enlarged, and the optical fiber grating strain sensor is high in sensitivity; the optical fiber grating strain sensor is simple in structure and high in sensitivity, is small and can be used for measuring small structural strain.