The invention discloses a method for producing an expansion agent after dry desulfurization of flue gas. According to the method, hot fuel gas is introduced into a reaction chamber, and then the hot fuel gas is enabled to flow upwards to be reversely mixed with a powdery calcium oxide expansion agent in the reaction chamber and subjected to desulfurization reaction with the powdery calcium oxide expansion agent, wherein the reaction chamber is a suspension preheater, the calcium oxide expansion agent serves as the desulfurizing agent, the hot fuel gas is introduced into the reaction chamber from one side of the lower part of the reaction chamber, and the powdery calcium oxide expansion agent is added into the reaction chamber from the upper part of the reaction chamber. Calcium sulfate generated from desulfurization reaction covers the surface of the powdery calcium oxide expansion agent, the obtained modified calcium oxide expansion agent is discharged from the lower part of the reaction chamber under the action of gravity per se, and the flue gas from desulfurization is discharged from the top of the reaction chamber after being purified. The calcium sulfate and calcium oxide containing mixture collected from the lower part of the reaction chamber can be sold and used as a composite expansion agent, and after being used for controlling atmospheric pollution, nearly 100% of the composite expansion agent can be recycled as a resource without causing secondary pollutants, so that the circulation in the economy can be realized.