A strong adsorption effect appears while ammonia passes through a tube or cell, and its polar molecules can easily stick to the wall surface. This results in low sampling quality, slow systemresponse, and low peak concentration, which largely affects the accuracy of gas-monitoring techniques. This work aims to develop a rapid and sensitive monitor system for ammonia concentration, and presents an evaluation method for ammonia desorption at the same time.The developed system of ammonia concentration monitoring makes use of a Quantum Cascade Laser (QCL) operating at 8.91 μm as its light source, along with the technique of Wavelength Modulation Spectroscopy (WMS) and multiple-pass cells of low-volume and long-pathlength. Particularly, we study the spectrum of ammonia absorption at 1 122.16 cm^-1 and implement a direct gas absorption monitor of ammonia under different pressures by fitting multiple direct absorption spectrum lines to its Lorentz curve, which analyzes the effects of gas pressures on the absorption spectrum within the specified spectrum range and lays the application foundation for wavelength modulation technology to obtain the 2f ammonia absorption spectrum curve in the target spectrum band.To suppress the system noise, improve its signal-to-noise ratio and further implement the measurement of ammonia wavelength modulation, the optimal pressure for ammonia gas monitor by wavelength modulation technology is hence set to be 0.8 atm.By monitoring the wavelength modulation of ammonia at different concentrations, it shows a perfect linearity between the amplitude of the obtained second-order harmonic wave and gas concentration, with a linear fitting of 99.50%. In the meanwhile, at the ammonia gas concentration of 0~100 ppm, the average error of the second-order harmonic wave is less than 3%, when the relative error is less than 1% at the gas concentration of 10 ppm~100 ppm, along with a sensitivity of 10.35 ppm/V. To evaluate the system stability, ammonia gas at the concentration of 6.25 ppm is prepared to make measurements when flowing through multiple-pass cells. By means of Allen variance analysis on the developed system, the system detection limit of 121.58 ppb is reached at its best integration time of 195 s.In addition, we conduct experiments on the absorption effects of ammonia under different tube materials and gas temperatures. For example, the system response time for detecting ammonia for the PU tube at the gas concentration of 6.25 ppm and room temperature is 138 s, while the system response time for PTFE is 19 s at the same gas concentration and a temperature of 200.The system developed in this work demonstrates excellent performance in the experiments of monitoring ammonia gas absorption in practice. On the other hand, it provides a novel method for studying ammonia absorption.