The invention discloses a method for preparing synthetic gas by utilizing flue gas. Carbon dioxide in the flue gas, oxygen in air and water vapor generated in the process are utilized for realizing the preparation of different proportions of synthetic gas through chemical-looping tri-reforming methane; meanwhile, energy is provided for chemical-looping reforming through chemical-looping combustion, so that the whole system realizes a complete self-heating process. The method disclosed by the invention not only can realize the preparation of the synthetic gas through tri-reforming reaction under the situation of no external energy, but also can realize the waste gas utilization of the carbon dioxide.