The boiler includes a blast-distribution plate under the fluidized bed. On the plate is mounted a blast cap which is connected below with a constant pressure blast chamber and then to the blower. Cinder cooling pipes are provided and buried in the fluidized bed. Over the blast plate is successively a fluidizing layer, a floating section and a U-shaped section. At the front and rear walls of the fluidized layer, charge port and spilling port are provided. An ash collector is fitted under the U-shaped section. The rear part of the U-shaped section is opposite to a bundle of convection pipes, flying ash separator, coal-saving device and air-preheater.