To investigate the demodulation accuracy of fringe counting method for optical fiber Fabry-Perot sensor, the cavity length demodulation error of the fringe counting method based on the composite algorism of crest and trough, and its influence factors that may cause demodulation errors were studied theoretically and experimentally. Results show that, the main influencing factors are interference ordersand location errors of spectral peaks and valleys. The variations of interference orders with the cavity length will cause periodic jitter demodulation errors, and the period equals a quarter of the center wavelength of light souce. Location errors of spectral peaks and valleys determine the magnitudeof demodulation errors, that is, the larger the location errors are, the larger the demodulation errors will be.