To improve the demodulation precision and efficiency of fiberoptic FabryPerot sensor, a series of optical path differences are calculated by using the peaks of the interference spectrum, and the rough demodulation result and the additional phase are obtained according to least square estimation such that the variance of the optical path differences is minimized. The spectral additional phase is corrected to obtain the compensated optical path difference .The final demodulation result is made up of the rough optical path difference and the compensation optical path difference. The simulation results show that the error of the algorithm is less than ±2.5 nm. The FabryPerot high temperature experiment results show that when the temperature is raised from room temperature to 1 000℃, the algorithm demodulates the optical path difference precision to 5.4 nm, the corresponding temperature precision is ±0.36‰F.S., and the average calculation speed is 400 times faster than that of FFTMMSE at the same condition. The algorithm is characterized by high precision and high calculating speed.