The invention relates to a system and a method for controlling oxygen concentration of a gasifying agent by a circulating fluidized bed gasification furnace, and belongs to the field of gas chemical industry. The system comprises a gasification furnace and is characterized in that an oxygen source, an oxygen line pneumatic cut-off valve, an oxygen line pneumatic regulating valve, an oxygen heaterand a pneumatic cut-off valve are connected sequentially; the oxygen line is divided into three oxygen main lines, which comprise an upper mixing area main line, a lower mixing area main line and a bottom main line, after passing through the pneumatic cut-off valve; a vapor line comprises a vapor source; the vapor source is connected with the above vapor access point; and a nitrogen line comprisesa nitrogen source and a proportioning nitrogen pneumatic regulating valve. The nitrogen source is accessed into each main line, so that the oxygen concentration of the gasifying agent is regulated timely and the operation elasticity of the gasifying agent is improved. The construction cost and the operation cost of equipment are reduced. Meanwhile, due to multi-nozzle gasifying agent distribution, the gasifying agent and the raw material can be completely mixed in the furnace, and local overtemperature scarring caused by the fact that a heat point is formed in the gasification furnace is avoided.