The present invention provides a simple method of mass producing high purity carbon nanocages. The method of preparing carbon nanocages comprises introducing an inert gas into a reactor equipped with a spray nozzle and raising the temperature of the reactor to a pyrolysis temperature of a catalyst compound. The method further comprises spraying a reactant including the catalyst compound and a carbon-containing compound into the reactor through the spray nozzle, wherein the ratio of a space velocity of the inert gas to a space velocity of the reactant is 100 or more.