The invention discloses a magnetic resonance imaging nanometer drug carrier, and a nanometer drug loading system and a preparation method thereof. The magnetic resonance imaging nanometer drug carrieris a triblock polymer nanoparticle, wherein the triblock polymer is PLGA-PEI-PEG, and has active groups on the surface, and the active groups comprise amino, hydroxyl and carboxyl. According to the present invention, the drug carrier can efficiently load a nuclear magnetic resonance imaging drug and an antitumor drug to make the antitumor drug specifically reach the tumor lesion site, such that the nuclear magnetic resonance positioning of the superparamagnetic ferroferric oxide nanoparticle at the tumor region can be achieved while the high-selectivity and low-toxicity treatment effect can be achieved, and the disadvantages of poor selectivity, strong toxic-side effect, easy drug-resistance generation and the like of the traditional cytotoxic drugs can be overcome; and the preparation method is simple and is easy to perform, the prepared triblock polymer nanoparticle can be stably stored in the aqueous solution so as to be easily stored, and various functional groups exist on the surface of the particle, such that the prepared triblock polymer nanoparticle can be easily subjected to surface modification or surface functionalization.