The invention provides an online balancing device for a substation accumulator battery, which is particularly for adjusting the balance of each battery in the accumulator battery. The whole device is composed of a voltage monitoring unit, an internal resistance test unit, a current monitoring unit, a temperature monitoring unit, a voltage balancing unit, a selector switch, an analysis unit, a main processing unit, a display unit, a communication unit, an alarming unit and the like. Monitoring on voltage, internal resistance and temperature of all batteries in the accumulator battery is realized; an automatic charging circuit unit and a discharging circuit unit are used for carrying out automatic charging and discharging on a battery with high voltage and a battery with low voltage; voltage balance of a single battery is achieved; the problems of overcharging and over discharging of the single battery are effectively solved; a balance adjustment function on the battery with the highest voltage and the battery with the lowest voltage in the accumulator battery is realized; the battery voltage is balanced actively; problems of over high voltage and over low voltage of single batteries can be eliminated; and a positive role is played to reduce advanced failure of the battery caused by inconsistent charging performance of the battery.