Based on UV spectrum measurement, a fast Fourier transform (FFT) online monitoring system of ammonia escape using high temperature extraction was introduced. The application effect and characteristics of this monitoring system were analyzed when used in the SCR de-NOx process with high temperature, high dust and other complex conditions. The results show that the repeatability of the instrument is 0.69%, and the correlation coefficient is 0.9992. In addition, the average loss of the sampling system is 0.15 μmol/mol which meet the requirements of ammonia escape detection in SCR de-NOx process and can operate stably for a long time.