The invention relates to a preparation method of a carbon-based magnesium ferrite composite membrane with a wave absorbing function. Industrial magnesium powder is ground for 3 h in a rolling and vibrating mill, then put into deionized water and subjected to an ultrasonic reaction at normal temperature under normal pressure for 5 hours, and white milky colloids are obtained; the prepared white milky colloids are dried at constant temperature and ground, and magnesium hydroxide nano-particles are obtained; iron perchloride is put into deionized water, a sodium hydroxide solution is added, the pH value of the mixture is adjusted to 7, ferric hydroxide precipitates are obtained, centrifugalized, subjected to freeze drying and ground, and ferric hydroxide nano-particles are obtained; the prepared magnesium hydroxide nano-particles and the ferric hydroxide nano-particles are mixed in the mass ratio being 1:2, the mixture is subjected to ultrasonic dispersion, dried, ground and calcined, and magnesium ferrite nano-particles are obtained; the magnesium ferrite nano-particles and a carbon nanotube are mixed in a mass ratio and subjected to ultrasonic dispersion and suction filtration, and the amorphous and flexible carbon-based magnesium ferrite composite membrane with the wave absorbing function is obtained. The process is simple, the cost is low, the product performance is excellent, and the preparation method is environment-friendly and pollution-free.