Fiber Fabry-Perot (F-P) sensors have been widely used in the detection of aircraft engines. Polarization cross-correlation demodulation is one of the most common methods for F-P cavity length demodulation. In a polarization cross-correlation demodulation system, the axial chromatic aberration of the optical system will cause the zero-order interference fringe shift and decrease the light intensity, which decreases the demodulation accuracy and the signal amplitude. The effect of axial chromatic aberration of the optical system on demodulation is analyzed. In order to reduce the influence of the axial chromatic aberration of the system on demodulation, an apochromatic optical system used in the polarization cross-correlation demodulation interrogator is introduced. The axial chromatic aberration along the wedge length is 1.9×10-4 m-1, and the maximum optical path difference is 0.021λ0 (λ0 is the central wavelength), which is far less than the F-P cavity length. For a certain demodulation system, the shift of the measured zero-order interference fringe is 2.85 μm, which is less than half of the pixel width of CCD, and the illuminance of any pixel of CCD is improved by 3.75 times.