The invention discloses a preparation method of a magnetic Fe3O4 nanometer particle, firstly, pentacarbonyl iron is mixed with 2-ketopyrrolidine used as a reaction medium to form a mixture which is reacted under a certain condition; secondly, precipitator is added into the prepared reaction liquid to lead the product (magnetic Fe3O4 nanometer particle) to be fully cemented out; at last, magnetic Fe3O4 nanometer powder having better magnetism controllability and room temperature paramagnetism can be obtained through vacuum drying. The preparation method of the magnetic Fe3O4 nanometer particle is simple, the size and the appearance of the product (magnetic Fe3O4 nanometer particle) are easily controlled, and the output is higher, thereby solving the problems in the prior art that the preparation process of the magnetic Fe3O4 nanometer particle is complex, the size and the appearance of the nanometer particle and the magnetism are uneasily controlled.