Applying secondary bonding to places where carbon fiber reinforced plastic (CFRP) structures are conventionally bolted leads to significant cost and weight saving. However, it has yet to be utilized due to difficulties in quality control. Lack of local pressure in bonding process has detrimental influence on the quality of the cured adhesive layer. Pressure detection is hence a key for controlling bonding quality, and a new quality assurance technique is urgently needed. This study established a fiber-optic-based pressure detection and quality assurance technique for secondary bonding process. Fiber Bragg grating (FBG) optical sensors were embedded in the adhesive layer and the reflected spectrum changes were monitored during the pressurizing stage to detect the applied pressure. Thermal residual strain after curing was also evaluated for further quality assurance. Good correlation was observed between the cure pressure, the CFRP quality and the FBG sensor response, confirming the validity of the proposed technique.