The invention discloses a photonic crystal optical fiber methane sensing device based on modular interference. The device is composed of a broadband light source, an optical circulator, a photonic crystal optical fiber methane sensor cladding an air hole coated polymer sensitive film, a testing air chamber, a switching valve, a quality flow controller, a spectrum analyzer and a computer, wherein the optical fiber methane sensor consists of a single mode fiber, a photonic crystal optical fiber and a welding area of the two fibers; the adopted photonic crystal optical fiber refers to an endless single mode solid core photonic crystal optical fiber; the polymer sensitive film refers to a polycarbonate resin methane sensitive film containing a cage molecule E-(OC2H5)6, and the inner wall of the cladding air hole of the photonic crystal optical fiber is coated with the film by virtue of an air pressure driving device; when the to-be-tested methane gas acts with the sensitive film on the inner wall of the cladding air hole of the photonic crystal optical fiber, the refractive index of the sensitive film changes, the reflection interference spectrum characteristic wavelength of the sensor is moved, the average movement amount shown in the description of the reflection interference spectrum characteristic wavelength before and after contacting the sensor with the methane gas is analyzed, so that the concentration of the to-be-tested methane gas can be obtained. The device disclosed by the invention has the characteristics of high sensitivity, high stability, high selectivity and the like.