Nitrogen oxides (NO_x) are important precursors of air pollution. NO_x in the atmosphere mainly comes from the combustion of fuels such as coal, oil, and natural gas. Large energy conversion equipment and motor vehicle exhaust are the main sources of NO_x emissions. In order to reduce NO_x emissions, China has promulgated various regulations to limit NO_x emission values. At present, flue gas denitrification is an important means to control NO_x emissions from large energy conversion equipment, including SNCR technology and SCR technology, etc. Among them, SCR technology has the advantages of high denitrification efficiency, low impact on the unit, fully automatic control of the system, easy installation and maintenance of equipment, etc., which has become the most widely used and mature flue gas denitrification technology. Ammonia (NH₃) is a common reductant for SCR technology. Although spraying excessive ammonia can improve denitrification efficiency, it is easy to cause ammonia slip. Ammonia slip from flue gas is a problem in many large energy conversion equipment SCR systems. A large amount of ammonia slip not only threatens unit operation and increases system maintenance costs, but also forms secondary pollution, which endangers the air environment and human health, so ammonia slip must be monitored and controlled effectively in real-time.Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology is an advanced trace gas detection technology with the advantages of high sensitivity, selectivity, and real-time online measurement, which is widely used in various pollutant gas detection fields and has become the mainstream technology for flue gas ammonia slip online detection. The modulation spectroscopy technique includes Wavelength Modulation Spectroscopy (WMS) and Frequency Modulation Spectroscopy (FMS). Compared with NIR, the intensity of absorption lines in the 9~10 μm band is 1~2 orders of magnitude higher, making it more sensitive for trace NH₃ detection. By adopting appropriate detection methods, analyzers of different wavelengths can basically meet the measurement requirements of flue gas ammonia slip sites. Although the mid-infrared laser source can provide high detection sensitivity, the high cost limits its wide range of applications, and the current commercial products are still mainly 1.5 μm laser sources.The unique physicochemical properties of NH₃ with strong adsorption and the complex environment in the flue make online detection of ammonia slips difficult. The complex measurement environment and strict emission limits place high demands on the anti-interference capability, stability, detection limits, and sensitivity of ammonia slip detection equipment. According to the application requirements of SCR denitrification sites, researchers have developed different ammonia slip detection methods, which currently include four main types: in-situ pair penetration, traditional sampling style, permeation tube style, and in-situ sampling style. The in-situ measurement method is simple and does not need to consider the adsorption of ammonia. Currently, most of the large thermal power unit denitrification ammonia slip online monitoring adopts the in-situ measurement style. However, influenced by the flue working conditions, the measurement accuracy is limited, and not suitable for the environment with high dust content. In contrast, the traditional sampling style has a stable measurement environment and can obtain a high signal-to-noise ratio spectral signal. However, the sampling process may lead to changes in flue gas components and has a certain lag, making it difficult to reflect the true value of the flue in real-time. The penetration tube style can avoid the extinction effect of dust and improve measurement accuracy, but requires frequent maintenance. The in-situ sampling style combines the advantages of in-situ and sampling measurement. The measured value is representative, the influence of dust particles can be avoided and the measurement accuracy is high, making it suitable for domestic coal-fired power station boilers with high dust content. Different ammonia slip detection methods have different advantages, disadvantages, and applicability. Currently, in-situ measurement equipment occupies the highest proportion, while the research and application of the remaining three methods are also in progress. Choosing the appropriate detection method according to different site environments is the only way to ensure the accuracy of ammonia slip measurement and the stability of system operation.By analyzing various types of flue gas ammonia slip online analyzers at home and abroad, it is found that their detection limits (or sensitivity) are basically less than 0.5 ppm to meet the site requirements, and the long-term stability of instrument operation depends on the actual situation. In recent years, the development of flue gas ammonia slip detection products based on TDLAS technology has accelerated, especially in China. With the increase of scientific research investment, China's independent research and development capability have gradually improved, and self-designed ammonia slip analyzers with better applicability and programmability will further meet the application needs of China. In my opinion, the in-situ sampling method developed from the traditional sampling method overcomes the shortcomings of the complex sampling system and poor real-time monitoring capability, which is more suitable for ammonia slip detection in domestic coal-fired power station boilers with high dust content, and the combination of calibration-free wavelength modulation, distributed multi-point measurement and other technical solutions can effectively improve the accuracy, real-time and representativeness of ammonia slip detection. Therefore, in situ sampling style ammonia slip analyzer will be further developed and applied.