In this paper, the direct absorption spectrum of methane 2v3 band R(9) manifold near 1.637 mm was detected The minimum detectable absorbance of 1.4×10-3 was achieved, while the minimum detectable absorbencies of 5.2×10-4, 1.1×10-4, 4.0×10-4 were obtained by use of harmonic detection for respectively in the condition of optimized modulation. There is a great improvement relative to the direct absorbance. Meanwhile the higher is the order of harmonic, the higher is the detection sensitivity.