The invention relates to a preparation method and an application of cationized magnetic carbon nano-tubes. According to the invention, the preparation of the cationized magnetic carbon nano-tubes and the applications of the cationized magnetic carbon nano-tubes as gene delivery carrier, thermo-sensitive agent, and magnetic resonance contrast agent in tumor treatment can be effectively solved. The method comprises the steps that: carbon nano-tubes are added into mixed acid; hydrogen peroxide is added; ultrasonic reaction is carried out, and pump filtration is carried out; a filtering cake is washed to neutral by using water; the filtering cake is dissolved into a mixed solvent; sodium acetate and ferric salt are added, and ultrasonic dissolving is carried out; a reaction is carried out, and the reaction product is added into anhydrous ethanol; ultrasonic processing and pump filtration are carried out, and the filtering cake is added into anhydrous ethanol; ultrasonic processing and pump filtration are carried out; the filtering cake is rinsed by using ethanol; the process is repeated 3 times, and ultrapure water is added; ultrasonic processing and pump filtration are carried out; ultrapure water rinsing is carried out; the above 2 processes are repeated 5 times; vacuum drying is carried out, and ultrasonic processing is carried out; the product is dispersed in a cationic liposome solution; ultrasonic processing is carried out 10 times; and centrifugation is carried out, and a supernatant is fetched, such that the cationized magnetic carbon nano-tubes are obtained. The preparation method provided by the invention is stable and reliable, and is easy to operate. With the nano-tubes, tumor treatment effect is greatly improved.