The invention relates to a hyperstable monodisperse fluorescent magnetic nano probe and preparation and application thereof. The fluorescent magnetic nano probe comprises a carrier and a light-sensitive agent, wherein the carrier is composed of carboxylated ferriferrous oxide magnetic nano particles, PEG and target molecules, and the light-sensitive agent is loaded on the carrier. The preparation method includes the specific steps that firstly, ascorbic acid is placed in an ferric salt solution, the pH value of the solution is adjusted to be 9-12, stirring and hydrothermal reaction are conducted, and an MNPs-COOH dispersion solution is obtained; secondly, the MNPs-COOH dispersion solution is centrifuged, then carboxyl groups of the MNPs-COOH dispersion solution are activated, PEG is added, mixing reaction is conducted, and an MNPs-PEG dispersion solution is obtained; thirdly, activated target molecules are added into and mixed with the MNPs-PEG dispersion solution for reaction, and an MNPs-PEG dispersion solution with the target molecules is obtained; fourthly, the MNPs-PEG dispersion solution with the target molecules and the light-sensitive agent solution are stirred and mixed and react, and a target product is obtained. The nano probe is used for dual-mode tomography in animal bodies. Compared with the prior art, the hyperstable monodisperse fluorescent magnetic nano probe has the advantages that targeting efficiency is high, residence time on tumor portions is long, and the preparation technology is simple.