Ammonia emission will cause harm to the environment and human health, so it is particularly important that the ammonia concentrations are measured with high precision. Off-Axis Integrating Cavity Output Spectroscopy (OA-ICOS), which has the advantages of high sensitivity and high response speed, is used to design a high-precision ammonia detection device. The gas absorption cell is composed of two high reflection mirrors with a reflectivity of 99.99%, and the base length of the optical resonator is 30 cm. Finally, an optical path of nearly 3000 m was realized. The Distributed Feedback Laser (DFB) with a central wavelength of 1528 nm is tuned to 6548.611 cm^-1 and 6548.798 cm^-1. The concentration of NH₃ is changed from 1×10 ^-5 to 5×10^-5 and is detected under an atmospheric pressure of 18.6 kPa at room temperature. The measurement results show that the linear fit R² between NH₃ concentration and signal amplitude can reach 0.99979. The Allan variance is used to analyze the experimental data, and the minimum detection limit of the system can reach 7×10^-9 at 103 s. The experimental results show that the detection device has good stability and high sensitivity, meets the demand for the high-precision detection of ammonia gas, and also provides technical experience for the domestic independent research and development of high-precision detection equipment for trace gases.