The invention relates to a preparation method of a water-soluble magnetic targeting carbon composite material and application of the water-soluble magnetic targeting carbon composite material in a medicine for diagnosing and treating tumor, and the water-soluble magnetic targeting carbon composite material can be used for solving the problems that the tumor curative effect is bad, the targeting is bad, and the toxic and side effect are great. The technical scheme of the water-soluble magnetic targeting carbon composite material is as follows: placing 10-20 mg of carbon material in 20 ml of mixed solution composed of ethylene glycol and diolacetal at a volume ratio of 1:19, stirring, ultrasonically dispersing uniformly, orderly adding 0.15-0.3 g of sodium acetate and 0.27-0.54 g of gadolinium chloride while stirring, sufficiently dissolving under ultrasonic, transferring to a reaction kettle, reacting for 10 hours at 200 DEG C, after the reaction, leaching and rinsing for 3-5 times by orderly using absolute ethyl alcohol and ultrapure water, obtaining a filter cake, performing vacuum drying for 24-56 hours at 20-60 DEG C to obtain the water-soluble magnetic targeting carbon composite material. The water-soluble magnetic targeting carbon composite material is good in biological compatibility, high in magnetic targeting, and small in side effect, can be effectively used for magnetic resonance contrast agents and medicines for diagnosing and treating tumors, and has great clinical significance.