Abstract: The external heat exchanger( EHE) is of great significance for the high-efficient operation of large circulating fluidized bed( CFB) boilers. In order to fully master the operation characteristics of external heat exchanger( EHE) in CFB boiler and eliminate the temperature deviation of EHE tube wall in ultra-supercritical CFB boiler at this stage,in this paper,the research status and key technology of the EHE were systematically summarized,and the design scheme of the EHE for ultra-supercritical CFB boiler was introduced. The results show that the wall temperature deviation of the EHE of 600 MW supercritical CFB boiler is very obvious,and the wall temperature distribution is saddle-shaped( the wall temperature of the tube panel in the center area is high and the wall temperature of the tube panel in the side wall area is low),which is determined by the uneven distribution of bubbles. The heat transfer coefficient of the EHE is closely related to the spatial position of the heating surface arrangement. When the speed of fluidization wind( Ug) of the EHE is 0.4 m/s,the heat transfer coefficient from the center of the bed to x/Xw≈0.6 is basically the same,while near the wall area,the heat transfer coefficient decreases significantly,and the side wall flow area accounts for about 25%-30% of the bed width. The arrangement of the tube bundle away from the side wall area can improve the inhomogeneity of the spatial distribution of the heat transfer coefficient,and the extreme deviation of the heat transfer coefficient is reduced from 15% to 6%. The design and calculation of the EHE of the 660 MW ultra-supercritical CFB boiler show that when the EHE is arranged with high reheating surface,the temperature difference between the middle diameter wall temperature of the high temperature reheater tube and the steam temperature can reach 58 ℃,which significantly restricts the safe operation of the EHE. Therefore,in the EHE of ultra-supercritical CFB boiler,all medium-temperature superheaters with relatively low steam temperature will be arranged. and TP347 H and TP347 HFG materials are used for the heating surface,which can meet the requirements and still ensure a certain margin. The arrangement of EHE tube is chosen to be parallel to the direction of ash flow in order to eliminate the local heat transfer unevenness caused by the side wall flow. In the design,it is considered to avoid the arrangement of the tube screen in the side wall area,and the distance between the tube screen and the side wall should be increased from 250 mm to more than 500 mm,which can improve the gas-solid flow and heat transfer behavior in the EHE to reduce the heat transfer deviation of the particle side. The design can control the furnace temperature and steam temperature effectively while ensuring the safe operation of the EHE.