The direct absorption spectra of methane at 2ν3 band R9 manifold are measured within the wavelength range of 1637.64～1637.85 cm-1 (in vacuum) by ECDL (external-cavity diode laser), and the absorption line strength corresponding to those are worked out through Voigt fitting. In addition, these results are compared with those that have been done by Jack S. Margolis with Fourier transform infrared spectrometer and the error is analyzed. The experiment and the calculation represent that the biggest error is 3.9%; the smallest error is 0.02% for eight absorption lines. Except the line 3 and the line 4, the results are comprehensible. The acquired data are used to remote sensing of the concentration of methane. The methods of the measurement and the calculation here also can be applied to measure the absorption line strength of CO, CO2, NH3 etc..