Disclosed herein is a mass production system and method of synthesized carbon nanotubes. The system is configured to completely open the reaction chamber to an outside during synthesis of the carbon nanotubes in the reaction chamber while allowing a specific gas to occupy a predetermined region within the reaction chamber, thereby blocking introduction of external air into the reaction chamber which is opened to external air. The system comprises a reaction chamber having at least one opening opened to external air, and at least one different-specific gravity gas occupying region filled with a different specific gravity gas having a different specific gravity from that of the external air to block the external air from being introduced into the reaction chamber through the opening, a carbon nanotube synthesizing unit positioned in the different-specific gravity gas occupying region to synthesize carbon nanotubes by the medium of a catalyst introduced thereto through the opening, a conveying unit to convey the catalyst to the carbon nanotube synthesizing unit through the opening, and a gas supply unit to supply the different specific gravity gas and a carbon source gas used for synthesizing the carbon nanotubes to the different-specific gravity gas occupying region and the carbon nanotube synthesizing unit, respectively.