The accurate monitoring of fiber Bragg gratings (FBG) during high stress changes and long-term service of unidirectional composites is important for the application of prestressed carbon fiber composite (CFRP) plate. The coupling of external-mounted bare-gate, externally-mounted substrate, and embedded bare-gate FBG sensors with CFRP plate is tested for the temperature sensing accuracy of the three coupling methods, and the process of stress loading up to 1000 MPa is monitored based on the prestressed CFRP plate reinforcement system. The experimental results show that all three coupling methods have good temperature sensing characteristics, but the stress sensing characteristics are significantly different from the temperature sensing characteristics. Among them, the embedded bare-gate sensor has the highest monitoring accuracy, and the linearity, hysteresis, and repeatability of its stress sensing are 0.9999, 0.25%, and 3.20% respectively, which can be applied to the reinforcement engineering of prestressed CFRP plate.