Tunable diode laser absorption spectroscopy (TDLAS) provides a reliable means for on-line detection technology of escaping ammonia.However,sampling measurement needs to overcome the effect of high temperature,dust and measuring hysteresis,and the NH3 concentration needs to be controlled at about 3 × 10-6 level.A new special Herriott cell was designed with the structure of the hollow path and the gas heating,and it can improve the measuring real-time.In order to evaluate the measuring accuracy of sample cell,the concentrations of 1 × 10-6 to 10×10-6 NH3 were detected，and their second harmonic spectrum were collected.Through the analysis of absorption peak and valley value,the fitting curve was obtained.According to the standard deviation of spectral curves,the detection limit of NH3 concentration is 0.22 × 10-6 in the Herriott cell.The results show that the Herriott cell with the high measuring accuracy and good real-time performance can meet the requirements of sampling measurement of trace detection of escaping ammonia,and it is suitable for online monitoring.