The invention relates to a preparation method of a gas diffusion electrode for producing formic acid by electrochemical reduction of CO2. The preparation method comprises the following steps: 1) adding anhydrous ethanol into powdery conductive carbon black till the powdery conductive carbon black is completely immersed, adding a polytetrafluoroethylene emulsion, performing reaction in a water bath, stirring till the formation of a lump, and then pressing by a roller to form a film, namely a diffusion layer film; 2) deoiling, acid-washing, tinning and drying a copper net to obtain a tinned copper net current collector; and 3) covering the diffusion layer film on the tinned copper net current collector, pressing by the roller, then cutting for formation, placing into a muffle furnace and calcining at the temperature of 3400 DEG C for 20-30min to prepare a target object. The preparation method provided by the invention has the advantages that according to the preparation method of the gas diffusion electrode, CO2 diffusion and mass transfer are strengthened by changing a CO2 transmission path and way of electrochemical reduction reaction of the CO2; and the preparation method has the advantages of simple process and low cost of raw materials, the prepared gas diffusion electrode has high mechanical strength, the electrochemical reduction efficiency of the CO2 is high, and the preparation method is suitable for engineering applications of electrochemical reduction of the CO2.