A symmetrical demodulation method is developed for the recovery of dynamic signals. Extrinsic Fabry-Perot interferometers (EFPIs) with different cavity lengths can be interrogated by a same demodulator. In the demodulation technique, three interferometric signals are introduced by selecting three specified laser wavelength, two of the three signals are symmetrical about the third signal. The dynamic signal is recovered by the proposed method from the three interferometric signals. EFPI sensors in a wide cavity length range (>1000 µm) can be demodulated without dead zone. The calculated amplitude error of the demodulated signal is less than 0.25% with the cavity length in the 20-1005 µm range. The proposed demodulation technique is adapted to the measurement of EFPIs with unsteady cavity lengths and unknown cavity lengths.