The invention provides a method for controllably synthesizing a carbon nano tube. The method is characterized by comprising the following step: performing auto-thermal reforming on an ester organic compound to synthesize the carbon nano tube. Particularly, the ester organic compound is used as a carbon source and then is subjected to auto-thermal reforming with a proper amount of water and oxygen under the action of a catalyst to synthesize the carbon nano tube. The auto-thermal reforming synthesis method provided by the invention can be used to prepare the carbon nano tube capable of controlling the diameter, the length, the shape and the arrangement. The preparation process is simple; the raw material is rich, and the cost is low; the industrial large-scale production requirement can be met; the prepared carbon nano tube has the advantages of high product quality, wide application range and wide market prospect.