A combined algorithm of amplitude normalization and minimum mean square error for the cavity length interrogation of fiber-optic Fabry-Perot sensors is proposed. An amplitude normalization method is introduced to realize the equiamplitude of both the reflected and the simulated spectral signals, which helps improving the interrogating accuracy. Significant increasing of interrogating accuracy compared with conventional algorithms is verified through numerical simulations. And, a 0.72 nm resolution of cavity length interrogation is achieved in real experiment.