The invention discloses a method for preparing Prussian blue by electrochemically converting magnetic beads and application of the method. The method comprises the steps that a magnetic bead aqueous solution is dispersed to be uniform through ultrasonic dispersion, citric acid and the magnetic bead dispersed solution are mixed and put on a rotating instrument for standing, and the steps of magnetic separation, supernate removing and washing with a phosphate buffer solution are sequentially conducted for three times; redispersion is conducted with a buffer solution, the obtained solution is dropwise added to cleaned gold magnetic electrodes, then the gold magnetic electrodes are put in a mixed solution of potassium ferricyanide and potassium sulfate to construct a three-electrode system to conduct multi-potential stepping, and the Prussian blue is obtained. According to the method for preparing the Prussian blue, operation is easy and controllable, the cost is low, the yield is high, the effect is significant when the Prussian blue is applied to hydrogen peroxide detection, and the wide application prospect is achieved.