An off-axis integrated cavity output spectroscopy (OA-ICOS) is established by using a fiber-coupled distributed feedback diode laser operating near 2 μm. Its performances are evaluated and optimized through experimental investigation via detecting the pure CH4 absorptions at different pressures. The reflectivity of the cavity mirror is measured to be 0.99865, which results in the effective total optical pathlength of up to 407.4 m based on a 55 cm cavity. It is shown that the OA-ICOS configuration can be used to obtain very long optical pathlength, leading to pretty high sensitive monitoring of atmospheric trace gases. Based on the developed OA-ICOS, the atmospheric CO2 measurements are made and its performance is improved by using the wavelet denoising approach. The CO2 absorption line at 4993.7431 cm-1 is used for measuring the concentration. The measured results are compared with the results obtaind by a commercial H2O/CO2 analyzer. Agreement and disagreement are briefly discussed, and the results show that the OA-ICOS is reliable for detecting the atmospheric trace gases. The limitation of the developed OA-ICOS and the further steps towards the improvement in precision and accuracy are also presented.