To solve the problem of the excessive cavity length demodulation error caused by the inability to determine the interference order due to the peak-to-peak method of demodulating the fiber-optic Fabry-Perot (FP) sensor.A peak-to-peak and interference order cavity length demodulation algorithm is proposed.The algorithm accurately determines the interference order of the selected peak by tracking the two farthest peaks in the reflection spectrum of the sensor,and then accurately solves the absolute cavity length according to the single-peak method.The accuracy of the cavity length solution is significantly improved by effectively eliminating the ambiguity of the interference order of the peak-to-peak method. Simulation analysis and experimental verification are carried out for FP sensors with cavity lengths of 30—100 μm.The simulated demodulation error is less than 0.7 nm,the experimental error is less than 1.4 nm,and the demodulation error is much smaller than that of the peak-to-peak method.