The invention relates to a preparation method of nano magnesium ferrite, which comprises the following steps: mixing FeCl3.6H2O and FeCl2.4H2O in a mol ratio of 1:1, dropwisely adding a 2 mol/L NaOH solution, and preparing magnetic Fe3O4 nanoparticles by a chemical coprecipitation process; putting the magnetic Fe3O4 nanoparticles into a muffle furnace, and roasting at 500 DEG C for 3-4 hours to obtain nano Fe2O3; putting magnesium powder in a beaker, adding deionized water, and putting the beaker in an ultrasonic cleaner to treat for 5-8 hours, thereby obtaining a white turbid liquid; putting the supernatant of the turbid liquid in a clean beaker, drying in a drying box at constant temperature, and grinding the product to obtain Mg(OH)2; roasting the nano Mg(OH)2 in a muffle furnace at 350 DEG C for 3-4 hours, and roasting the Fe3O4 nanoparticles in a muffle furnace at 500 DEG C for 3-4 hours; respectively obtaining nano MgO and nano Fe2O3; and taking the nano Fe2O3 and the nano MgO, mixing the nano Fe2O3 and the nano MgO in a mol ratio of 1:1, adding deionized water, treating in an ultrasonic cleaner for 10-12 hours to obtain a red turbid liquid, sucking the supernatant of the turbid liquid into a clean beaker, drying the beaker in a drying box at constant temperature, and grinding the product to obtain the nano magnesium ferrite. The nano magnesium ferrite prepared by the method provided by the invention has the advantages of low cost, high purity, uniform appearance, environmental protection and no pollution.