Using a distributed feed back (DFB) tunable diode laser as the injection light source, absorption spectrum around 1.573 μm with high signal-to-noise ratio (SNR) and sensitivity is obtained by using off-axis integrated cavity output spectroscopy and scanning the laser and cavity simultaneously. The typical sensitivity is 4×10-8 cm-1 (SNR=2,1s integrated time). It is not valid to fit the integrated cavity output spectrum with the nonlinear least square method, and it will make the self-broadening coefficient broadened as about 2.39 times of the real value. The air-broaden coefficient approaches to the real one. Absorption coefficient fit is necessary to obtain the correct parametersof spectral line width. Theoretical analysis and experiment demonstration prove the conclusion.