The invention discloses a preparation method of a cage shaped active carbon for flue gas desulfurization, and belongs to the field of active carbon synthesis. The preparation method comprises the following steps: taking cheap and easily-available loofah sponges as the raw material, soaking loofah sponges in a sodium hydroxide solution, then bleaching loofah sponges by calcium hypochlorite, carrying out fermentation, drying fermented loofah sponges in a quick-cooling and quick-heating mode by using a refrigerator and a high temperature baking oven, subjecting loofah sponge fibers to a de-crystallization treatment under the assistance of ball milling so as to improve the reaction activity of loofah sponge fibers, making loofah sponge fibers into pulp taken as the raw material, taking cyclohexane and trichloromethane as the mixed oil phase, tween 60 as the emulsifier, and ammonium persulfate as the initiator, mixing the raw material, mixed oil phase, emulsifier, and initiator with a water phase, carrying out reactions under a heating condition to generate cage shaped fibers, and finally carrying out high temperature carbonization to obtain the cage shaped active carbon for flue gas desulfurization. The obtained caged shaped active carbon is a novel active carbon material, there are many holes on the surface of the active carbon, the cavity in the active carbon is large, the fluid mechanic performance is excellent, and the provided active carbon has a wide application prospect in flue gas desulfurization therefore.