A compact instrument based on the off-axis integrated-cavity output spectroscopy(ICOS) technology was developed for sensitive measurements of gas mixing ratios(ammonia in air) at room temperature by using fiber-coupled distributed feedback(DFB) diode laser operating at 1.531μm. The absorption line of ammonia at 6528.764cm^-1 was chosen for trace detection. The mirrors' effective reflectivity R² of 0.9969 was first calibrated by carbon dioxide under this condition, and the avity 35.8cm in length as an absorption cell could yield an optical path of presumably 115.46m. As a result, a minimum detectable concentration of approximately 2.66ppmv(S/N～3) at the total pressure of 100 torr was obtained. Then the lock-in a mplifier was added in the system to acquire the econd armonic signal by ombination of wavelength modulation technology, which could better suppress background noise and improve the signal-to-noise ratio, and a detection limit of 0.293ppmv(S/N～3) was achieved eventually. This work demonstrated the potential of the system for a range of atmospheric species sensing in the future.