Ammonia (NH3) is one of the main precursors of secondary fine particulate matters in the atmosphere. The accurate measurement of NH3 concentration plays an important role in the monitoring and protection of atmospheric environment. Although the cost of a near-infrared laser is relatively low, there always exist some problems such as the H2O, CO2 gas interference in the environment and the limited absorption path when it is used for the NH3 concentration measurement. In order to reduce the H2O and CO2 interference in the environment, we screen out the absorption line with a central wavenumber of 6521.97 cm -1, which is used for the measurement of the trace NH3 concentration in the atmospheric environment. Moreover, this spectral line is not affected by the CO2 absorption in the environment and its overlap range with the H2O absorption line is small under the condition of low pressures. The spectral absorptivity of NH3 can be accurately extracted through multi-peak fitting. A measurement device based on cavity ring-down spectroscopy is built with a distributed feedback laser, in which the ring-down cavity is composed of a pair of high reflectivity mirrors with a reflectivity of up to 99.996%, the empty ring-down time is about 96 μs,and the effective absorption path is about 1.6×10 4 m. The trace NH3 concentration in the atmospheric environment is measured with this device, and the results show that the detection sensitivity of this measurement system can approach 3.9×10 -10.