The invention provides a preparation method of potassium ferrate. The preparation method comprises the steps of firstly, preparing a mixed solution of KClO and KOH: performing direct-current electrolysis on an electrolyte containing KCl as a main salt in a diaphragm-free electrolytic bath to obtain a KClO solution, performing direct-current electrolysis on an electrolyte containing KCl as a main salt in a diaphragm electrolytic bath to obtain a KOH solution, absorbing chlorine generated by an anode to obtain a KClO solution, and mixing cathode solutions of the diaphragm-free electrolytic bath and the diaphragm electrolytic bath to obtain a KClO/KOH mixed solution; then, adding a trivalent iron source compound into the mixed solution to obtain K2FeO4. The preparation method can realize continuous operation, and the solution containing KCl after deposition of K2FeO4 can be recycled to realize discharge without wastewater.