Well casing shall be protected by impressed current cathodic protection in order to prevent corrosion. When cathodic protection system is put into operation, protective potential cannot meet relative requirements from beginning to end. Through calculating protection current requisite amount and field testing, it is found that due to lack of anticorrosive coating on external wall of well casing and protective current may produce cathode voltage cone around the whole well casing, soil potential around the well casing will greatly reduce and even becomes less than that in distant soil, the reference electrode installed near the well casing will be affected by the cathode voltage cone around the well casing, which will make protection potential displayed in device more positive than its actual protection potential. Therefore, the reference electrode for well casing cathodic protection shall be installed outside the cathode interference area.