For the accurate cavity-length demodulation of fiber-optic Fabry–Perot (FP) sensors, a combined correlation method based on the fundamental cross-correlation and a higher-order one is proposed, simulated, and experimentally verified. By extending the reflection spectrum eightfold through continuous frequency-doubling three times, cross-correlation using both the original and eightfold spectra, and determination of the main peak of the fundamental cross-correlation coefficient function with the assistance of the eighth-order cross-correlation coefficient, the cavity-length demodulation resolution for fiber-optic FP sensors can be significantly improved even when the spectral bandwidth of the source is limited or the cavity length is relatively short. A cavity length resolution better than 1.8 nm is achieved for an FP sensor with a cavity length of ∼162 μm. The proposed demodulation method can effectively reduce the bandwidth requirement of the light source for the cavity-length extraction of fiber-optic FP sensors, particularly those with relatively short cavity lengths.