The invention relates to the technical field of metal soft magnetic material manufacture, in particular to a preparation method of an iron-nickel-molybdenum metal magnetic powder core with the magnetic permeability mu = 400. The preparation method comprises the steps of 1, weighing material; 2, gas atomization pulverizing; 3, granularity combination; 4, heat treatment; 5, activation treatment; 6,once insulating coating; 7, twice insulating coating; 8, compression molding; 9, core annealing; 10, immersion spraying. According to the preparation method of the iron-nickel-molybdenum metal magnetic powder core, before the insulating coating, the surface of the magnetic powder is activated by using an activator, the coating effect is increased, the formed coating layer is more uniform, and theprepared core is not easy to break; organic reagent is not used, and the preparation process is greener and more environmentally friendly; a high temperature release agent is adopted, the heat treatment temperature is high, the forming pressure is low, the magnetic permeability of the magnetic core is improved; the whole method is simple in process, easy to implement and strong in operability.