The invention relates to a method for preparing formic acid through electrochemical catalytic reduction of carbon dioxide, and belongs to the technical field of carbon dioxide recycling. In the method, a proton exchange membrane separates an electrolytic tank into a cathode chamber and an anode chamber, organic solvent / ionic liquid / water mixed solution in which a large amount of carbon dioxide is dissolved is injected into the cathode chamber, and aqueous solution containing supporting electrolyte is injected into the anode chamber; and after an electrolysis power supply is connected, the carbon dioxide undergoes electroreduction reaction on the cathode to form the formic acid. By the method, the organic solvent / ionic liquid / water mixed solution with the advantages of good conductivity, low viscosity, high capacity of dissolving the carbon dioxide, wide electrochemical window, and low use cost can be obtained, and when the carbon dioxide is electrically reduced in the mixed solution, the current density in the electroreduction reaction of the carbon dioxide can be improved and the electrocatalytic activity and long-time stability of a cathode material are improved.