The invention relates to a method for preparing high-purity chromate by using ferrochromium, wherein the method comprises the steps: providing an electrolytic device including an electrolytic tank, adouble-cathode device, a double-anode device, an insulating partition plate, a plurality of settling tanks and an electrode; carrying out an electrolytic reaction by a self circulating mode, automatically controlling the temperature at 20 DEG C-60 DEG C, and controlling the electrolyte solution circulating reaction time according to the formula 1 described in the specification, to obtain a high-concentration chromate solution; and inducing the high-concentration chromate solution into a reaction kettle, carrying out a hydrothermal reaction, under the pressure of 2 MPa-10 MPa, heating to the temperature of 180 DEG C-300 DEG C, carrying out a reaction for 2 h-12 h to obtain a hydrothermal reaction slurry, carrying out solid-liquid separation to obtain the high-purity chromate purification solution, and carrying out evaporation concentration, cooling crystallization and filtration separation to obtain the high-purity chromate product.