Abstract: In general,ultra-supercritical circulating fluidized bed( CFB) boilers are equipped with external heat exchangers( EHE) to increase the heat transfer area and heat transfer. In order to study the startup process characteristics of the external heat exchanger of CFB boiler,so as to provide the basis for wide load operation and flexible peak regulation of CFB,The dynamic characteristics of startup process of EHEs of a 660 MW ultra-supercritical CFB boiler were studied,by establishing the dynamic mathematical models of CFB boiler with EHEs and simulating and analyzing the cold startup process and the hot startup process of EHEs. Before the cold startup and the hot startup,the furnace temperature is 776 ℃,while the temperature of EHEs is 50 ℃ and 660 ℃ respectively. At the 30 th minute,the conical valve is opened and EHEs start to work. Due to the low temperature of EHEs in the cold startup,the conical valve is only 5% open in order to avoid overheating of the EHE pipe. Such limitation does not exist in the hot startup,and the conical valve can be open up to 30%. In order to show the change caused by one variable,only the opening degree of the conical valve is different in the cold startup process and the hot startup process,and the slag discharge quantity is automatically controlled in other active control quantities to maintain the stable furnace pressure. In the cold startup process,heat released from EHEs changes from negative value to positive value: EHEs absorb heat from the working medium( medium temperature superheated steam) first because the initial temperature of EHEs is lower than that of the working medium; when the temperature of EHEs is higher than that of the working medium,EHEs release heat to the working medium. In the hot startup process,EHEs always release heat to the working medium all the time,because the initial temperature of EHEs is much higher than that of the working medium. Therefore,the flow rate of working medium should be maintained at the set value and above before cold startup or hot startup,otherwise the condensation or sudden temperature rise of working medium at the startup moment will lead to the drastic temperature change of pipe,resulting in pipe burst and other accidents. When EHEs reach energy balance after cold or hot startup,the furnace temperature decreases in varying degrees,because the furnace releases less heat and EHEs release more heat accordingly,under the condition that the input energy of CFB boiler remains unchanged. In the hot startup process,the circulating ash flowrate and heat discharge of EHEs are large,because the opening degree of the conical valve is 30%,resulting that the furnace releases less heat than that of the cold startup and the furnace balance temperature is lower than that of the cold startup. In the cold startup process,the temperature rise of the external heat exchanger is higher. The conical valve is only 5% open in order to avoid the high rate of temperature rise on the wall of the heat exchange surface,resulting that the characteristic time of the temperature rise curve of EHEs is obviously larger than that in the hot startup process.