Through analysing the spectrum of the cascade optical fiber Fabry-Perot pressure sensor composed with three reflecting surfaces, the lower envolope of the spectrum can be used to demodulate the sensor′s cavity length. A novel algorithm based on fitting of interference-order is proposed. Simulation analysis is carried out with the cavity length of F-P2 variating between 160 μm to 215 μm. It is shown that the results precision can be improved using the novel method at the different signal to noise ratio (SNR), and is not impacted by sensor′s cavity length and the optical spectrum resolution ratio at the same SNR. The experiment is also carried out, and the calculation results show that the residuals of the least square curve fitting is 0.012 μm, which verify the effectiveness of the new algorithm.