The demodulation method of fiber Bragg gratings (FBG) based on long-period fiber gratings (LPFG) was studied. The transmission spectrum of LPFG was used as the edge filter to demodulate the FBG, and the demodulation performance mainly depended on the transmission spectrum depth of LPFG. The influence of CO₂ laser writing technique on the transmission spectrum was analyzed in detail, and the LPFG with transmission spectrum depth of 25 dB was manufactured and used as the FBG demodulation device. The demodulation system was built with the FBG pasted on the surface of the metal strain sheet. The static and dynamic response of the sensing system was tested. The experimental results show that the proposed method has the advantages of simple production process, low cost, good linear dynamic range and higher sensitivity.