Spectrum of fiber bragg grating(FBG) sensor modulated by double long period grating(LPFG) is proposed in the paper. Double LPFG consists of two LPFGS whose center wavelengths are the same and reflection spectrum of FBG sensor is located in linear range of double LPFG transmis sionspectrum. Based on spectral analysis of FBG and double LPFG, reflection spectrum of FBG modulated by double LPFG is obtained and studied by use of band-hider filter characteristics for double LPFG. An FBG sensor is attached on the surface of thin steel beam, which is strained by bending, and the center wavelength of FBG sensor will shift. The spectral peak of FBG sensor modulated by double LPFG is changed correspondingly, and the spectral change will lead to variation in exit light intensity from double LPFG. Experiment demonstrates that the relation of filtering light intensity fromdou ble LPFG monitored by optical power meter to center wavelength change of FBG sensor is linear and the minimum strain of material(steel beam) detected by the modulation and demodulation system is 1.05 με. This solution is used in impact monitoring of optical fibre smart structure, and FBG sensor is applied for impulse respon sesignal monitoring induced by low-velocity impact, when impact pendulum is loaded to carbon fiber-reinforced plastics(CFP). The acquired impact response signal andf ast Fourier transform of thesignal detected by FBG sensor agree with the measurement results of eddy current displacement meter attached to the FBG sensor.From the results, the present method using FBG sensor is found to be effective for monitoring the impact. The research provides a practical reference in dynamic monitoring of optical fiber smart structure field.