The invention discloses a flue gas decarbonization system and a flue gas decarbonization method for two-step regeneration of carbon dioxide. The flue gas decarbonization system is formed by coupling an alkylol amine and methanol mixed solution adsorption system with a two-step carbon dioxide regeneration system, wherein when the two-step carbon dioxide regeneration system works, two steps of removing methanol before regeneration and regenerating alkylol amine carbonate are carried out. In the step of removing methanol before regeneration, flue gas waste heat is utilized to provide a heat source to remove the methanol in rich liquor after adsorption; the regeneration process only aims at the alkylol amine carbonate; the energy consumption of sensible heat and evaporation heat are avoided; and the steam consumption of the thermal regeneration process is reduced. Two regeneration towers alternately implement removal of the methanol and regeneration of the alkylol amine carbonate by two sets of valve equipment, so that the continuous operation of a trapping system is ensured. According to the invention, both the flue gas waste heat and the condensation heat of methanol steam are utilized, the temperature of the regenerated carbon dioxide is reduced, the flue gas decarbonization of a power plant is efficiently implemented, and the flue gas decarbonization system and the flue gas decarbonization method for two-step regeneration of carbon dioxide have the characteristics of cleanness, high efficiency, energy saving, high decarbonization rate and the like.