Based on the infrared absorption spectrum of methane, a dual-bandpass filter is designed by the spectral splitting. With the adjustment of ion source beams, the refractive index of the ZnS film layer is improved and the background noise is eliminated. The fabrication difficulty of aperiodic narrow-bandpass filter films is reduced by the introduction of controllable thin film layers. The experimental test and analysis show that the prepared dual-bandpass filter possesses the transmissivity of 89.25% at 3.31 μm and 81.97% at 7.67 μm, respectively, and the goal for improving the sensitivity of detecting systems is achieved.