The invention relates to a preparation method of carbon nano-tube (CNT) / polyimide composite material, which comprises the procedures that: firstly, the carbon nano-tubes (CNT) conduct an reaction in concentrated nitric acid and concentrated sulfate, as well as ultrasonic under 50 to 60 DEG C for 2 to 3 hours and reflux in 40 to 80 DEG C water for 5 to 6 hours; then the carbon nano-tube is cooled, cleaned and dried. The acidized carbon nano-tubes (CNT) react with isocyanate under ambient temperature and then go through ultrasonic, cleaning and drying. Secondly, organic solvent is added and nitrogen is connected for deoxidation protection; diamine monomers are added with mechanical agitation. Equimolar dianhydride monomers are added in batches, with icy agitation for 4 to 6 hours. Thirdly, Alkyl based carbon nano-tubes (CNT) are treated with ultrasonic in organic solvent for 36 to 48 hours to generate suspension of the carbon nano-tubes (CNT); the suspension is added to polyamide acidsolution with icy agitation for 3 hours. Fourthly, the mixed solution is poured on a flat plate and solvent is removed during 1 hour under 60 DEG C. The solution conducts heat insulation for 1 to 2 hours under 100 DEG C, 150 DEG C, 200 DEG C, 250 DEG C, and 300 DEG C respectively to achieve the carbon nano-tube (CNT) / polyimide composite material. The invention has the advantages of simple technology of preparation, low cost and non-pollution for the environment; meanwhile thermal stability of the resultant composite material is enhanced. The resultant composite material can be widely used in the fields of mechanics and chemical engineering.