A circulating fluidized bed boiler comprises a main combustion chamber, two auxiliary combustion chambers, a secondary furnace and the like, wherein the two auxiliary combustion chambers are symmetrically disposed on the two sides of the main combustion chamber; the two sides of the upper portion of the main combustion chamber communicate with the two auxiliary combustion chambers correspondingly to form flue gas and material channels; the secondary furnace is arranged at the rear ends of the main combustion chamber and the two auxiliary combustion chambers, and the bottoms of the auxiliary combustion chambers communicate with the bottom of the secondary furnace to form flue air and material ducts; the bottoms of the auxiliary combustion chambers and the bottom of the secondary furnace incline towards one side of the secondary furnace; the bottoms of the auxiliary combustion chambers are provided with secondary air chambers and a plurality of secondary air ports, and the secondary air chambers communicate with the bottoms of the auxiliary combustion chambers through the secondary air ports; an ash discharge hopper and an ash discharge pipe are arranged at the bottom of the secondary furnace; and the ash discharge pipe is connected with a material back-delivery device, and the included angle beta between the horizontal direction and a back-delivery pipe from the junction to the material back-delivery device of the main combustion chamber is not smaller than 45 degrees. Through the M-type and overall N-type flue gas flow path structure characteristics of the main combustion chamber and the auxiliary combustion chambers, the flue gas flow path is long, the combustion efficiency is high, and meanwhile the boiler is compact in structure and small in occupied area.