The invention discloses a simple preparation method of a Fe3O4 magnetic nano-material and provides a novel method for a preparation technology of the magnetic Fe3O4 nano-material. And present preparation technologies are improved. According to the technological process in the invention, ammonium ferrous sulfate and triethanolamine are prepared into a mixed solution at the mol ratio of 1:0.5-1; the reaction solution is stirred at room temperature for 3-5 minutes; the reaction solution turns grass green from blackish green and is then put into a program control microwave reactor; the microwave power is 300W; the refluxing reaction temperature is 90 DEG C; the stirring rate is 500 r/min; the microwave heating time is 6-8 minutes; after microwave heating, the reaction solution is naturally cooled to room temperature, and a precipitate is filtered and air-dried to obtain the black Fe3O4 magnetic nano-material. The method is simple and rapid. The nanometer grain size is between 5nm to 20nm.It shows through a magnetic property test that nanocrystals have paramagnetism. In comparison with traditional Fe3O4 nano-powder preparation methods, the technology in the invention is simple, requires no inert atmosphere protection, and requires cheap and easily-obtained raw materials. The method provided by the invention is easy and simple to operate, produces no environmental pollution, is suitable for industrialization, and has a good application prospect in the fields of catalysis, biomedicine, energy and environment and the like.