Since the carbon atoms from the hydrocarbon gases are the unique resource of carbon deposition in alkali vapor laser, precise measurement for the gas pressure in a stand-off way will be helpful to depict the carbon contamination phenomenon quantitatively. Based on the Fourier transform infrared spectroscopy, a method for detecting the methane pressure is presented. In the method, the peak (3.369 μm) on the right side of the main peak is chosen to be the characteristic peak. The effects of the alkali cell tilt on the accuracy are also researched. The experiments results show that the maximum deviations are 0.055 kPa and 0.057 kPa when only methane is in the cell or the volume ratio of methane and helium filled in the cell is 1∶3. The deviations will be higher than 0.075% if the tilt angle is higher than 0.0035 rad. The measurement accuracy of the methane pressure can characterize the carbon contaminating problems, and provide a reference for investigating the mechanism of carbon contamination in the alkali lasers.