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Objective: The ultra-small superparamagnetic iron oxide particles were used as the carrier to construct and characterize nuclear magnetic/fluorescent molecular probes through polyethylene glycol-linked fluorescent Cy5.5. Methods: Ake a small amount of Cy5.5-nhs fluorescent powder and dissolve it in dimethyl sulfoxide （DMSO） solution. After centrifugation and ultrafiltration of PEG ferric oxide particles, resuspend the nanoparticles with phosphate buffered saline （PBS） to change the pH of PEG ferric oxide nanoparticle solution. The configured cy5.5 fluorescence was added to Fe3O4particles, incubated in a constant temperature shaking table, and the larger iron ions and unbound fluorescence were removed through a centrifugal filter. After standing, the hydrated particle size and zeta potential were detected, the relaxation rate was detected by newwheat nuclear magnetic resonance, the cytotoxicity was detected by CCK-8 experiment, and the uptake of the probe by cells was observed by laser confocal microscope. Results: The Cy5.5-PEG-Fe3O4probe was successfully synthesized. Transmission electron microscope （TEM） showed that the diameter of the probe was 16.8 ±2.4nm,the water and diameter of the nanoparticles were 43.4±17.6 nm, and the Zeta potential was -18.0 m V. The relaxation rate is 39.5 m M-1·s-1and the slope is 0.98. The results of cytotoxicity experiments show that it is slightly toxic to cells, and the toxicity is dependent on the concentration. Laser confocal results show that this probe can be taken up by cells smoothly. Conclusion: This experiment successfully synthesized Cy5.5-PEG-Fe3O4probe. [ABSTRACT FROM AUTHOR]