NOVELTY - Preparation of polythiophene derivative nanocomposite fiber at low temperature, comprises adding mixed solution of concentrate sulfuric acid and concentrated nitric acid to a flask, adding multi-wall carbon nanotube and ionic water and dispersing by ultrasonic wave, and carrying out condensing and refluxing; cooling, washing, drying and grinding; adding multi-wall carbon nanotube to calcium chloride solution and dispersing; adding thiophene, dispersing, adding anhydrous ferric chloride or iron sulfate, and stirring at - 10 to - 30 degrees C; and filtering, washing and drying.    USE - Method for preparing polythiophene derivative nanocomposite fiber for use in the fields of transistor, conducting material, solar battery material, photoelectric apparatus, sensor and electromagnetic shielding.    ADVANTAGE - The polythiophene derivative synthesized at low temperature has higher molecular orientation and better conductivity in comparison with the polythiophene derivative synthesized under normal condition. Because the thiophene derivative monomer has good molecular orientation at low temperature, the nanocomposite fiber has high conductivity, which is 134-187 S/cm by the test. The preparation method protects environment, saves energy, has simple process and low preparation cost, and is suitable for batch production.    DETAILED DESCRIPTION - Preparation of polythiophene derivative nanocomposite fiber at low temperature, comprises:    (A) adding mixed solution of concentrate sulfuric acid and concentrated nitric acid at volume ratio of 3:1 to the single-mouth flask; adding 100-500 mg multi-wall carbon nanotube and 50-120 ml ionic water to the flask; dispersing the mixture for 10-30 minutes by ultrasonic wave; heating the mixture to 120 degrees C by oil bath; and carrying out condensing and refluxing for 2-5 hours;    (B) cooling the product to room temperature, and filtering the product; using deionized water to wash the product several times till that water phase is changed to neutral; drying the product for 5-10 hours in oven at 80 degrees C, grinding the product, and obtaining acid-treated multi-wall carbon nanotube;    (C) adding 100 mg of the multi-wall carbon nanotube to 150-250 g of 20% of calcium chloride solution; dispersing the product for 30-90 minutes by ultrasonic wave, and obtaining uniform solution;    (D) adding thiophene to the solution in step (C); dispersing the solution for 10-30 minutes by ultrasonic wave; adding 20-50 g anhydrous ferric chloride or iron sulfate; filling nitrogen, carrying out stirring for 10-72 hours at - 10 to - 30 degrees C, where the catalyst is ferric chloride or iron sulfate; and the mass ratio of the catalyst to thiophene derivative monomer is 15-30; and    (E) after reacting, filtering the product; using deionized water to wash the solution till no chlorine ion or sulfate ion exists in the solution; and drying the solution for 10-24 hours in the vacuum drying box at 50 degrees C.