Abstract: Ammonia is one of the most common toxic gases in livestock and poultry environment. Its high concentrations can pose a potential health threat to humans, plants, and animals. Traditional active detection methods increase energy consumption, heat buildup may affect detection system performance, and are not suitable for livestock and poultry environment where circuit wired connections are limited. With the gradual transition from traditional to smart agriculture, radio frequency identification (RFID) technology has been widely used to integrate device sensing and wireless communication, due to its lightweight, low-cost, and non-line-of-sight readability. In this study, a passive RFID sensor simulation model was designed by using the high-frequency structure simulator (HFSS) software. A split-ring resonator was employed to operate at a center frequency, which was adjusted according to the length of the open gap. The additional flexibility was provided rather than the closed-loop resonant structure. According to the HFSS simulation model, a physical RFID tags were fabricated by screen-printing technique based on polyethylene terephthalate and polyimide substrates, and carbon nanotubes with high surface area were selected as ammonia-sensitive materials. The surface morphology and nanostructure of carbon nanotube materials were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The sensor resistance was measured due to the contact of ammonia molecules on the surface of the sensitive materials, mathematical model of passive detection was established to analyze the sensing mechanism. Once an ammonia molecule came into contact with the surface of a carbon nanotube, some electrons or holes were used to change the carrier concentration, thus leading to the varying resistance of the carbon nanotube. Antenna frequency or impedance mismatch was found during adsorption between ammonia and free carriers on the surface of carbon nanotubes, thereby affecting the backscattering signal domain of RFID, the detection process of RFID sensing tags was simulated by analyzing the variation of transmission coefficients. Furthermore, a radio-frequency test system for ammonia testing in laboratories and livestock environment was built. The transmission gain of the tag can be calculated by detecting the transmission coefficient offset and amplitude change. According to the tag power reflection coefficient, return loss and phase analysis, the resonant frequency of the tags varied by 270 MHz under 0-18 mg/L ammonia ambient. The detection efficiency of the tag was easily affected by carbon dioxide, temperature and humidity factors, due to manual cutting, substrate deformation, environmental interference and other factors. There was a deviation about 0.05 GHz between the resonance frequency of the physical tag and the simulated resonance frequency, the sensing tag's sensitivity was about 15 MHz·L/mg, and the maximum reading distance was 24 cm. The sensing tag has obvious advantages in terms of service life and response time compared to commercial ammonia sensors. The tag sensor can be expected to fully meet the passive detection needs of ammonia. This finding can provide a reliable theoretical and practical basis for the passive detection of ammonia from agricultural sources. Further research can also be conducted to select the sensitive materials or suppress the interferences of radio frequency links in the future.