The principles of frequency-division multiplexing technique of fiber grating Fabry-Pérot (F-P) sensors were theoretically analysed and the restrictions on the length of F-P cavity for signal processing were given. The numerical simulation results shown that sensors with different cavity lengths are represented by different spectral fringe frequencies. To ensure that signals in the frequency domain will not overlap, the difference in the cavity length of any two fiber grating F-P sensors must exceed the length of fiber Bragg grating. The further experiment and simulation analysis also show that the change of measurands (i.e. temperature) will only cause the whole spectra shift without shape change, and the corresponding signal in the frequency domain produces only phase shift, so that the demodulation method based on Fourier transform for common fiber Fabry-Perot (FFP) sensor would be inapplicable here. According to this specialties of the frequency-division multiplexing signal, an autocorrelation analysis method has been suggested for the signal demodulation of this sensor system.