This study introduces a new methane measurement method based on the degenerate four-wave mixing effect by coating a methane gas-sensitive film on the inside of two specific cladding pores in a photonic crystal fiber. The method accurately detects methane gas in real time by establishing a direct relationship between the movement of the Stokes and anti-Stokes spectra in optical fiber and the change in methane concentration. The experimental results show that the sensor sensitivity is closely related to the gas-sensitive film thickness and the pump wavelength. At the same pump wavelength, the sensing sensitivity increases as the thickness of the gas sensing film increases. Under an unchanged thickness of the gas-sensitive film, the closer the pump wavelength is to zero dispersion wavelength, the higher the sensing sensitivity. The gas sensing sensitivity corresponding to the Stokes and anti-Stokes spectra can reach -4.87 nm/% and 2.83 nm/%, respectively, after the structural parameter optimization. The related measurement principle is also applicable to the accurate measurement of other gases and the composition analysis of mixed gases.