This study presents a high-sensitivity detection method for a specific biological molecule using a graphene-based optical biosensor. The use of functionalized magnetic nanoparticles as biological molecule carriers, which can be immobilized on the graphene surface under a magnetic field, improves the graphene surface-modification process. The effectiveness of our proposed method was successfully verified in an Ochratoxin A detection experiment. We obtained a detection limit of 0.01 ng·mL ^-1 for Ochratoxin A, a good response in the range of 0.01--5 ng·mL ^-1, and a good function of the biomolecule specificity. We believe that the proposed method can expand the scope of applications of graphene-based optical biosensors, simplify the biomolecule-sensing process, and improve the sensitivity of graphene-based optical biosensors.