Aiming at a structural stress monitoring system, it is proposed that using the principle of fiber grating sensing to measure structural stress.A F-P tunable filter fiber grating demodulation system based on fiber etalon is designed. Using the stability of the etalon to calibrate the F-P filter to eliminate the interference of the ambient temperature on the F-P filter. The driving source of the F-P filter is designed by using the DDS principle, and the resolution of triangle wave can be less than 0.42mV, ensuring that the final resolution of the demodulation system reaches 1pm. The photoelectric conversion circuit is designed to realize the conversion and amplification of the weak light signal, and finally it is converted into a voltage signal that is easy to collect and display. Finally, the demodulation system designed in this paper can achieve a strain measurement of ±3000με, and the measurement accuracy can reach 2.59%.