The invention relates to the technical field of boilers, in particular to a circulating fluidized bed (CFB) boiler low-nitrogen combustion emission system and a transformation method thereof. The low-nitrogen combustion emission system comprises a flue gas recirculation system, an SNCR denitration system and a biomass fuel blending combustion system. Through transformation on a feeding system, anair distribution system, a secondary air system, the flue gas recirculation system, a separation system and the SNCR denitration system and adjustment on a combustion process, a temperature of a CFB along a material flow is equalized, effective nitrogen reduction in a reduction zone and efficient desulfuration in an oxidation zone are organically combined, a partial air-coal ratio to ensure a burn-off rate is uniform, and graded air supply for secondary air jet three-dimensional layout and good fuel adaptability for an overall low-oxygen combustion process are realized. Through efficient low-nitrogen combustion in a furnace, original emission of NOx reaches below 100 mg/Nm<3>; and through optimization on the SNCR denitration system, the removing efficiency of more than 80% is realized on the basis of efficient low-nitrogen combustion in the furnace, and the NOx emission is lowered to below 20 mg/Nm<3>, meeting the requirement on super-low NOx emission.