Aiming at the traditional calibration process of grounding current monitor for outer cable sheathing, the interference of background leakage current in field environment is not considered under the laboratory reference conditions. A zero-flux principle and space- After the injection of a standard signal combination of on-line calibration method, without removing the relevant equipment, the background leakage current to eliminate the impact of the calibration process, the monitor calibration results of the uncertainty of a detailed analysis and assessment based on monitoring The working principle and testing method of the instrument verify the rationality of the credibility and uncertainty of the calibration result through the method of passing comparison.