Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology, with the advantages of high selectivity and high accuracy, provides a reliable technology means for ammonia (NH₃) on-line detection of the environment. There are many factors affecting the measurement accuracy of TDLAS system. Among them, temperature and pressure are the two basic influence conditions. First, the present paper introduces the effects of temperature change on NH₃ concentration detection. Between -10 ℃ ~ 50 ℃,using Hollow Waveguide (HWG) cell to detect the NH₃ of 50 ppm, the second harmonic spectra were obtained, which show that the amplitude of the second harmonic signal will decrease with increasing temperature. When the pressure changes from 0 kPa to 100 kPa, the amplitude of the second harmonic signal will decrease with the increasing of the pressure. According to the above results, the experience formula of temperature and pressure correction was raised. The maximum relative error of the 50 ppm concentration NH₃ after temperature correction is -5.5% at different temperature condition. The long time monitoring results show that the system could adapt to on-line monitoring after correction.