The optical wedge plays a vital role in the cross-correlation demodulation system of optical fiber Fabry-Pérot(F-P) sensors. A mathematical model for the light intensity reflected from the optical wedge is established based on analyzing results of the factors that cause the interference optical phase differences between the optical wedge and the F-P cavity of optical fiber F-P sensors according to the cross-correlation demodulation principle. The light intensity reflected from the optical wedges with different reflectivity combinations are numerically simulated and a simplified mathematical model for the light intensity reflected from the optical wedge is established by comparing the light intensity reflected from the optical wedge with those from the F-P cavity with the same reflectivity combinations. The analysis results of errors between the simplified mathematical model and the mathematical model indicate that the error of the simplified mathematical model for optical wedges is less than 0.6% when the optical wedge angle is less than 0.1° and the product of two surfaces′ reflectivity is not more than 9%. The simplified theoretical model of the optical wedges lays a theoretical foundation for establishing optical fiber F-P sensor systems based on the cross-correlation demodulation principle and further improving the accuracy of the demodulation system.