The infrared methane sensor detects the concentration of methane according to Beer-Lambert Law. What’s more, the absorption coefficient is an important parameter in Beer-Lambert Law, which varies greatly under the influence of temperature and pressure. The change will lead to the error of concentration measurement. Therefore, it is of great significance to study the variation law of methane absorption coefficient under different temperatures and pressures for the design of high-precision infrared methane sensors. In the literature reports, the experimental data of measuring methane concentration affected by the environment are generally obtained and then processed mathematically to compensate and correct the measurement error. Based on the theory of molecular spectrum analysis, the methane with 3 016.49 cm^-1 wave number is taken as the research object, and the methane data in the HITRAN database are used to design a Python program to call the HAPI function to fit and calculate the variation law of methane absorption coefficient with temperature and air pressure, and the variation law of methane absorption coefficient is verified by Fourier infrared spectrometer. Results show that at 3 016.49 cm^-1, water molecules (the influence of humidity) have little influence on the methane absorption coefficient, which can be ignored. Temperature and air pressure have certain effects on the absorption coefficient. When the air pressure is 1 atm and the temperature increases from -10 ℃ to 50 ℃, the absorption coefficient of methane decreases, and the relationship between the absorption coefficient and temperature is linear. When the temperature is 273.15 K, and the air pressure increases from 0.6 atm to 1.2 atm, the methane recovery coefficient increases and the relationship between the absorption coefficient and the air pressure is linear. Finally, the formula of absorption coefficient with temperature and air pressure is fitted, k(T, p)=53.65(±3.24)-0.114 6(±0.010 7)T+21.07(±0.95)p. Methane standard gas concentration is 1.01%, 2.00%, 3.51%, 5.06%, respectively, which are introduced into a short optical path quartz gas cell with a diameter of 2.5 cm and a length of 8 cm. The absorbance of methane was obtained from Fourier infrared spectrometer by changing the gas pressure and temperature. Due to the influence of the instrument resolution in the laboratory, if the methane concentration is directly inverted by absorbance, the error is large. In this paper, the absorption coefficient ratio to absorbance is used to judge the correctness of absorption coefficient fitting. The results show that the ratio of absorption coefficient to absorbance is fixed when the concentration is fixed and the air pressure and temperature change, further confirming the correctness of the calculated fitting methane absorption coefficient changing with temperature and pressure.