Battery online monitoring system is widely used in power plants and substations. Lack of an effective on-site calibration of its performance after the battery online monitoring system is installed makes it an urgent problem to evaluate its performance during the acceptance test and normal operation. This paper proposes a possible solution which is the ‘calibrated battery & calibrating-testing device’ method based on the internal resistance measuring methods given by Thevenin model and GB/T19638.1 (IEC 60896-22 MOD). The battery-internal-resistance ration En of the calibrated battery and on-site measuring values is less than 0.191 and meets the requirements given by ISO/IEC 17025 standard, which shows that this method is a practical way to calibrate battery-internal-resistance measuring device on-site and helps to increase the operation and maintenance standard of the battery.