During the process of monitoring trace gases of the atmosphere based on tunable diode laser absorption spectroscopy and long open path technology, signal amplitude changes relatively large due to the diurnal light intensity variation, atmospheric visibility changes and other factors, and the signal amplitude may exceed the range of A/D conversion after amplification. Automatic gain modular was designed to amplify signals in the laser detection system, and monitor ammonia concentration continuously of Quangang Petrochemical Parks in Fujian Province. The monitoring result shows that the ammonia concentration of the Petrochemical Park has a diurnal variation cycle: the concentration is low in the morning and night, and high in the noon, ammonia mainly comes from fuel combustion and industrial emissions. The problem of large signal fluctuation in the signal acquisition process was solved. The function of the automatic gain modular were verified and the stable and continuous trace gases detection was achieved.