A method for collecting carbon dioxide from flue gas of a power plant. The method includes: 1) mixing an organic amine, an ionic liquid, and water to obtain an aqueous solution of a composite absorbent; 2) spraying the aqueous solution of the composite absorbent into the flue gas to allow the flue gas flowing upwardly to fully contact with the downwardly sprayed aqueous solution of the composite absorbent and to allow CO2 in the flue gas to react with the composite absorbent to yield a solution rich in A.CO2 and B.CO2, where A represents the organic amine and B represents the ionic liquid; allowing the solution rich in A.CO2 and B.CO2 to stand and clarify to form different liquid layers; 3) thermally decomposing the mixed solution rich in A.CO2 and B.CO2 after heat exchange to release the chemically bound CO2.