Abstract: Circulating fluidized bed (CFB) boiler with low load operation is facing technical challenges, including high coal consumption amount, low main steam temperature, and extremely difficult satisfaction to ultra-low NO_x emission regulation of 50 mg/m³, which all severely limited the industrial application of the deep peak-shaving technology of CFB boiler. A new technological route was proposed and adopted in a 240 t/h CFB boiler, with two fluidized self- preheaters (heat capacity: 24 MW) installed in the left side wall and right side wall, respectively. By coupling particle coal supplied into the main furnace of the CFB and pulverized coal supplied into the preheater, the upper temperature of the main furnace was increased, the reduction efficiency of NO_x with preheated fuel was enforced, and the technical problems was expected to be solved for CFB boiler with low load combustion. The investigation results show that the self-preheating temperature of pulverized coal in the preheater is higher than 800 ℃, and the stable operations separately in 25% load and 20% load were achieved. Also only with NH₃ injection into the furnace, the NO_x emission were 38.2 mg/m³ in 25% load and 47.5 mg/m³ in 20% load, satisfying the regulation of ultra-low NO_x emissions. With the ultra-low load, the temperature in the dense zone of the main furnace is over 750 ℃, creating a beneficial condition for high-efficient combustion of coal particles supplied into the CFB. When the boiler load being 25%, the outlet temperature of the main furnace is over 670 ℃, forming a technical scheme of stable, high-efficient, and clean combustion with ultra-low load. The experimental results provide important support for research development and industrial application on the deep and flexible peak-shaving of CFB boiler.