The invention relates to a synthesis method of lectin-iron core gold shell nanoparticles for tumor MRI / CT dual-mode diagnosis, and belongs to the technical field of medical diagnostic devices. The synthesis method comprises: a, adding an iron salt solution comprising ferrous chloride and ferric chloride according to a Fe II / Fe II molar ratio of 0.5 to a 1.5 M NaOH solution according to a volume ratio of 1:10 in a dropwise manner, vigorously stirring, collecting the produced black precipitate by using a magnet, carrying out water washing, collecting, adopting tetramethylammonium hydroxide to wash, dissolving the precipitate, diluting the obtained ferric oxide nanoparticle solution to 1.1 mM with water, adding sodium citrate, stirring, and sequentially and alternately adding hydroxylamine hydrochloride and chlorauric acid to obtain gold coated ferric oxide core-shell structure nanoparticles; and b, carrying out a reaction of lectin and NHS-PEG-S-S-PEG-NHS, carrying out a reaction of the PEG in a K2CO3 solution for at least 4 h, and continuously carrying out a reaction of the Fe2O3@Au nanoparticles and PEG-SH5000 for more than 1 h. The lectin-iron core gold shell nanoparticles have beneficial effects of small particle size, simple synthesis method, good stability, long blood circulation time, and no immunogenicity. In addition, specific lectin binding target molecules can be screened based on different tumor cells.