The invention relates to a preparation method of a magnetic nanomaterial, which comprises the following steps: preparing a ferrite: dissolving the ferrite solution into deionized water, stirring till dissolving under protection of inert gas to obtain the ferrite solution; preparing a magnetic carrier material: heating the ferrite solution in step (1), rising temperature to 57-70 DEG C, adding an oxidant into the ferrite solution during stirring, continuously stirring after the oxidant is added, removing a liquid phase after reaction is ended, washing to obtain nano Fe3O4 particles; sealing the nano Fe3O4 particles: hermetically preserving the nano Fe3O4 particles after the nano Fe3O4 particles obtained in step 2 are dried. According to the preparation method, peroxide or hyperoxide of alkaline metal is used as the oxidant, a common ferrite is used as an iron source, the magnetic carrier material with good dispersion effect is fast and efficiently prepared at one step under the condition that the deionized water is taken as a solvent, the reaction is short in time and can be completed in 0.5hour, and time is saved.