The invention belongs to the field of electric power application engineering, and particularly relates to a testing method of water permeability of silicone rubber. The testing method of the water permeability of the silicone rubber comprises steps as follows: the silicone rubber and sheet iron are bonded and then are encapsulated with a plastic cylinder and paraffin, and an electrolytic tank is formed; an electrolyte solution is added to the electrolytic tank, an electrochemical AC impedance test is performed, and capacitance values of the silicone rubber at different moments are obtained; a diffusion coefficient D of water in the silicone rubber is calculated. The method is combined with the electrochemical AC impedance test to analyze and determine the water permeability of the silicone rubber, has the advantages of being high in pertinence, wide in coverage, simple to operate and accurate in determination result, has greater guiding significance in evaluation of water permeability and aging degree of a silicone rubber part of a composite insulator, and can effectively prevent accidents of a high-voltage power transmission line.