Time-domain laser absorption spectroscopy (TDLAS) with the advantages of high selectivity and high accuracy provides a reliable means of on -line detection technology for escaping ammonia (NH₃).The concentrations from 10×10－6 to 100×10－6 of NH_3were detected with the TDLAS system in the two multi- pass cells of White cell and Herriott cell,and their second harmonic spectra were collected .The two kinds of cell measurement results are compared,and the advantages and disadvantages are analyzed and compared.Experiment results show that the no ise of the Herriott cell is higher than that of the White cell.According to the standar d deviation of spectral curves,the detection limit of NH_{3concentration} is 1.21×10－6 in White cell,and is 1.61×10－6 in Herriot cell.The resu lts show that the Herriott cell is more suitable for the harsh environment,and the system can meet the requirements of trace detection of escaping ammonia,suitable for on-line monitoring.