A remote sensing system was developed based on direct-sun infrared absorption spectroscopy, which was running continuously in Hefei, Anhui. The total atmospheric transmittance was measured from the solar absorption spectrum and modeled by line-by-line calculation. The column concentrations of methane and oxygen were inversed from measured total atmospheric transmittance by nonlinear least squares spectral inversion algorithm. The column concentration of oxygen was used as internal standard function to obtain column-average dry-air mole fractions of methane and the precision was better than 2%. The column-average dry-air mole fractions of methane, from 12:00 to 15:00 on September 25, 2012, were compared with methane results observed by Japanese greenhouse-gases satellite over this site during the same period, showing that the relative deviation was less than 1%. Obviously, this system and algorithm are an effective way to detect methane column concentration with higher precision.