Mechanical incomplete combustion heat loss is the second biggest heat loss in boiler operation, mechanical incomplete combustion heat loss is mainly determined by the carbon content of the fly ash in the boiler. The carbon content in fly ash of boiler is an important index to reflect the combustion efficiency of boiler, real-time, accurate detection of fly ash carbon content is an important means of improving boiler operating efficiency and reducing power generation cost. By the actual measurement results of the carbon content of fly ash in power plant in China, the necessity of on-line monitoring of carbon content in fly ash from the perspective of economy and environmental protection is described, the characteristics and advantages and disadvantages of existing detection methods in this paper, a brief introduction to the current flying ash measuring instrument. A complete solution is put forward for the problems of common sampling method in power plant, the development trend of carbon technology in the future fly ash is described, and it has a certain reference value for the detection of carbon content in fly ash.