We report an off-axis integrated cavity absorption spectroscopy instrument based on distributed feedback (DFB) laser and applied it for measuring trace moisture in high purity nitrogen gas and standard methane. The light emitting from the laser centered at 1392 nm is directly focused into the cavity formed with two high reflectivity mirrors, separated by a distance of 60 cm. The output signal of the cavity is focused into the photoelectric detector. The mirror reflectivity is calibrated by that of standard methane detected at different pressures. The reflectivity of the cavity mirror is measured to be 0.99843±0.00003 at 1.599 kPa. The H2O absorption lines at 7189.344 cm-1 and 7185.597 cm-1 are respectively used for measuring trace moisture in high purity nitrogen gas and standard methane. The optimum noise-equivalent absorption coefficients of 1.65×10-8 cm-1 and 3.75×10-8 cm-1 in 200 s are achieved, respectively, which corresponded to the volume fraction detection limits of 2.28 ×10-6 and 4 ×10-6. The volume fraction detection limit can be increased by about 24 times and 19 times, respectively, reaching about 95×10-9 and 210×10-9 via the strongest absorption line at 7181.155 cm-1. The volume fraction detection limit of the system can be up to the level of 10-12 if the mirrors with a higher reflectivity and the strongest absorption line are used.