In order to achieve gas component identification and concentration quantitative detection，an F-P tunable filter with a wavelength range of 9~12.6 microns for gas detection is designed in this paper. The specific optical elements of the MEMS microstructure that affect its optical performance are given special attention in the design. To solve the problem of Si substrate transmission at wavelengths above 9 μm，a high-transmission film composed of ZnS and BaF₂ is designed，with a transmission rate of 97% or more in the wavelength range of 9~12.6 μm. The movable reflector and fixed reflector are both made of a high-reflective film composed of PbTe and BaF₂，achieving a reflection rate of up to 98% in the designed wavelength range. The design is verified by simulation using TFCalc and MATLAB software，proving the rationality of the design. The final results show that the design can provide a reference for the design of tunable filters for long-wave applications，which can be widely used in gas sensors，environmental monitoring，medical diagnostic equipment，etc.，especially in miniaturized and integrated sensor systems.