The high-pressure absorption spectrum at 6046.96 cm^-1 of methane was obtained with the measurement experimental platform. The empirical mode decomposition algorithm was used to filter out the detection noise caused by window deformation at high-pressure. Consequently， the overall root-mean-square error （RMSE） of absorbance signal was reduced by 3.87 times， and the residual error of absorbance signal fitting was lower than ±1% by using Lorentz line-type fitting algorithm. These studies showed that the absorption line width increased with the pressure increasing， and the mutual broadening coefficient of nitrogen-methane molecules at high-pressure was calculated as 0.0631 cm ^-1atm ^-1. Moreover， the absorption line appeared a "red shift" phenomenon as the pressure increasing， and the pressure shift coefficient of nitrogen-induced was calculated as -0.00848 cm ^-1atm ^-1. Therefore， a method of concentration inversion at high pressure was proposed by the linear relationship between the detection wavelength， pressure， and pressure shift coefficient. In conclusion， the research on spectrum broadening characteristics in high-pressure environment lays foundation for spectrum detecting in industrial environment.