The invention relates to a low-ratio biomass circulating fluidized bed boiler and a combustion method thereof, relating to a circulating fluidized bed boiler and a combustion method thereof and solving the problems of difficult feeding, grate slagging, bed material coking, tempering and output reduction, and the like existing in the operation of traditional grate furnaces and circulating fluidized bed boilers. A middle air chamber corresponds to the middle of a dense-phase area at the lower part, lateral grid plates are arranged between a lateral air chamber and the dense-phase area at the lower part, and a medium-temperature steam-cooled cyclone separator is arranged at the joint between a flue shaft at the tail part and a health outlet of a boiler body, respectively communicated with the flue shaft and the health outlet and communicated with the dense-phase area at the lower part through a material returner. The method adopts a negative-pressure feeding manner so that the fluidization air velocity of a high-speed area of the dense-phase area at the lower part is 4-5 m/s, the fluidization air velocity of a low-speed area is 1.5-2.0 m/s, the temperature of the dense-phase area at the lower part is 650-780 DEG C, and the material circulation ratio is below 10. The invention is suitable for the biomass circulating fluidized bed boiler for an electric power plant.