The invention relates to a fiber grating strain sensor. An elastic strain sheet is provided with a longitudinal shaft and is symmetrical along the longitudinal shaft, the elastic strain sheet comprises a pair of symmetrical U-shaped beam elastic structures, a flat plate for connecting the U-shaped beam elastic structures, a groove used for placing fiber grating and located on the longitudinal axis of the elastic strain sheet, a pair of rectangular holes used for a tail fiber to penetrate and located at two ends of the groove, a temperature compensation device, a fixed device and a sheath fixing device. The fiber grating is arranged in the groove, two ends of the fiber grating are fixed at corresponding positions by using glass welding materials through a point welding mode, the tail fiber penetrates through the rectangular holes and are located on the other surface of the elastic strain sheet, a sheath is used for protecting, and the fiber grating strain sensor is formed by the elastic strain sheet, the fiber grating, the sheath and an auxiliary device. The fiber grating strain sensor is simple in structure, large in measuring range, convenient to install and capable of eliminating interference of temperature on the sensor effectively.