A method for preparing carbon nanotubes for synthesizing carbon nanotubes, fabricating carbon nanotube films and electronic devices is provided. The method for preparing carbon nanotubes can repair the defects in side walls of carbon nanotubes under stable condition easily and prepare carbon nanotubes of excellent properties. The method utilizes uric acid solution or ammonia water to treat carbon nanotubes after acidifying the carbon nanotubes by refluxing with nitric acid. The treatment temperature is, for example, 25° C.˜90° C., and treatment time is at least two 2 days. Preferably, the carbon nanotubes are treated with thionyl chloride solution before being treated with uric acid solution or ammonia water.