The invention relates to a preparation method of flat-shaped high-frequency wave-absorbing magnetic powder. The preparation method comprises the following steps of, step 1, selecting materials, specifically, ferromagnetic metal powder and ball milling solvent, and selecting organic solvent; and step 2, ball milling, specifically, selecting a wear-resistant ball as a ball milling medium, mixing theferromagnetic metal powder with the ball milling solvent, a dispersing agent and other auxiliaries in a planetary ball mill, wherein the ball material ratio is 10:1-20:1, the mass percent of dosage of the dispersing agent and other auxiliaries of the ferromagnetic metal powder is less than or equal to 1%, and then carrying out ball milling for 2-6 h under the condition that the rotating speed ofthe planetary ball mill is greater than or equal to 400 r/min, carrying out preliminary flattening of the ferromagnetic metal powder, adjusting the rotating speed of the planetary ball mill to less than 200 r/min, reducing the impact force of the ball milling medium on the flat powder, reducing the breaking rate of the ferromagnetic metal powder, continuing the ball milling for 1-8 h, and carryingout discharging and drying so as to perform annealing treatment at 500-800 DEG C under the protection of reducing atmosphere of argon, hydrogen and nitrogen for eliminating stress. According to the preparation method, the flattening rate can be improved, the flattening time can be saved, and the performance of the magnetic powder can be improved.