Abstract: Accurately, sensitively and rapidly monitoring of ammonia slip after the flue gas denitration to avoid ammonia secondary pollution of the environment is one of the focuses in industrial and environmental fields. This research chose the absorption line of ammonia molecules near 1.53 m as the goal line, studied the ammonia slip in situ monitoring method of high temperature flue gas through TDLAS technology and wavelength fast scanning technology, and designed the corresponding open-path measurement system. This research analyzed the temperature influence on measurement in the high temperature environment, studied the temperature correction method, designed an ammonia concentration accurate inversion algorithm which avoided calibration in flue-site and obtained the maximum relative detection error which is 1.5% by laboratory experiments. The engineering practicability and reliability of the algorithm of the system is proved by industrial field installation and operation. It will provide the effective technical support in industrial denitration process monitoring and the flue gas emissions in safety in our country.