The peak value of reflection spectrum can be measured and used as characteristic value for the central wavelength by sampling the reflection spectrum of FBG. With this method, a great deal of data need to be collected and processed, and the demodulation speed of the system is decreased. A simple but effective method is put forward to solve the problem. The analysis on FBG reflection spectrum indicates that the FBG reflection spectrum is very symmetrical. Based on this characteristic, the method to average the double edge of a wavelength is proposed for demodulating the central wavelength of FBG. The experiment proved that the demodulation system using this technique can process the reflection signal of FBG easily and obtain the central wavelength of FBG accurately. With this technique, the speed of demodulation for FBG sensing signal was improved.