Abstract: The design principle of a 660 MW high efficiency ultra-supercritical circulating fluidized bed (CFB) boiler was discussed, while its main performance parameters were predicted. The boiler adopts two parallel furnaces, and each furnace has an independent air supply system. External heat exchangers are used in the scheme, while panel superheaters and panel reheaters are arranged in the upper furnace. To reduce the deviation of steam temperature caused by incomplete symmetry of both the temperature and mass flow of flue gas and bed material in the two independent loops, the air supply rate, coal feeding rate and bottom ash discharging rate are controlled, steam mixture headers are used, while the flow direction of inlet flue gas in the backpass is changed. Results show that the design scheme with double furnaces can well solve the problem of temperature deviation, and ensure the positive flow characteristics in waterwalls even at low mass flux. Additionally, the auxilary power comsuption can also be reduced due to lower water pump head.