1) Preparing graphene oxide by using a Hummers method;

2) Weighing and dissolving the graphene oxide in deionized water to obtain a first solution, performing an ultrasonic treatment on the first solution, and then adding a predetermined amount of a strong oxidant for being fully dissolved to obtain a second solution, wherein the strong oxidant is a mixture of at least one of potassium persulfate, hydrogen peroxide, and hypochloric acid;

3) Adding 400-1,000 μL of a basic compound to the second solution to obtain a resulting solution, and then refluxing the resulting solution for 7-12 h at 70-120° C.;

4) After the resulting solution is refluxed, performing a purification and a drying on the resulting solution to obtain the graphene quantum dots;

5) Preparing the graphene quantum dots into a third solution with a predetermined concentration, wherein the graphene quantum dots are pure, and then adding a predetermined amount of a gadolinium chloride solution to perform a chelation reaction under predetermined conditions to obtain a solution after the chelation reaction; and

6) Performing a purification and a drying on the solution after the chelation reaction to obtain the graphene quantum dots-gadolinium ion chelate.