The present invention provides a method for preparing high purity carbon nanotubes, in which, when carbon nanotubes are prepared by the recombination of carbons generated from a carbon source in the presence or absence of a catalyst as in arc-discharge, laser ablation, chemical vapor deposition or vapor phase continuous growth method, and the like, water of 1 to 2000 wt% based on a carbon source is added in the reaction system to prepare high purity carbon nanotubes. According to the present invention, the addition of water in the reaction system suppresses the soot formation resulting from the pyrolysis of a carbon source itself and induces the oxidation or reduction of the formed soot by water, and thereby high purity carbon nanotubes can be prepared economically and easily.