Abstract: This article studies the debond and crack propagation of carbon fiber reinforced plastic (CFRP) T-joints under tensile load. The numerical model for the T-joints provided the sensitive areas on which fiber Bragg grating (FBG) sensors could be fixed. FBG sensors along with a high speed camera were used to monitor the appearance and propagation of debond. The results show that the damage initiates at the T-joints filling area. The cracks propagate along two directions: the horizontal direction, i.e., the glue layers between the L prepreg and the skin and the vertical direction, i.e., the glue layers between two L prepregs. The propagation of cracks ultimately causes the structure failure. The changes of the FBG central wavelengths succeed in recording the appearance, accumulation and propagation of damage under non-visual conditions, which can help alarm the inner damage of the structure and indicate the crack propagation paths.