By annealing at high temperature (850~950℃), the fiber Bragg grating can be erased at high temperature and grow again to form a regenerated fiber Bragg grating, which can work stably in a high temperature environment of more than 1 000℃. However, the mechanical strength of the regenerated fiber Bragg grating after annealing at high temperature is significantly lower than that of common fiber Bragg gratings. In this paper, the axial stress of fiber Bragg grating and the change of quartz molecular components in fiber Bragg grating are studied and analyzed by single mode quartz fiber experiment. The results show that compared with the unannealed fiber Bragg grating, the compressive stress at the fiber core decreases by 80 MPa, and the tensile stress at the cladding far away from the fiber core gradually decreases by 22 MPa. At the same time, with the increase of oxygen content in the atmosphere of hot annealing, the regenerated fiber grating SiO₂ generated after annealing gradually increased, and the proportion increased from 52.99% to 69.92%. Although SiO₂ has a high density and its mechanical strength is greater than Si₂O₃, the brittleness of the regenerated fiber grating after hot annealing still increases. Therefore, it is inferred that the change of components has no significant effect on the increase of brittleness of regenerated fiber Bragg grating, and the main reason for the increase of brittleness is the stress relaxation caused by high temperature. This paper provides a reliable theoretical and experimental basis for improving the mechanical properties and solving the brittleness problem of thermal regenerated fiber Braggg ratings.