The invention relates to a synthetic method of a lanthanide metal loaded fullerene nanomaterial water-soluble derivative and an application of the water-soluble derivative. The synthetic method and the application achieve the preparation of the lanthanide metal loaded fullerene nanomaterial water-soluble derivative and a magnetic resonance contrast for treating a tumor. The synthetic method comprises the steps of placing amination-modified fullerene in a mixed solvent A, stirring and conducting ultrasonic treatment to allow amination-modified fullerene to be dispersed uniformly, adding a solvent B and a substance C sequentially during stirring, placing in a reaction kettle after sufficient ultrasonic dissolving, adding absolute ethyl alcohol after a reaction, conducting ultrasonic treatment and suction filtration, washing a filter cake with absolute ethyl alcohol, transferring the filter cake into a beaker, adding absolute ethyl alcohol, conducting the ultrasonic treatment and the suction filtration, washing the filter cake with absolute ethyl alcohol, repeating for three times, transferring the filter cake to the beaker, adding ultrapure water, conducting the ultrasonic treatment and the suction filtration, washing the filter cake with the ultrapure water, repeating for five times, and conducting vacuum drying to obtain the lanthanide metal loaded fullerene nanomaterial water-soluble derivative. The synthetic method is simple; and the obtained water-soluble derivative is good in use effect, and can serve as the magnetic resonance contrast effectively during tumor treatment.