The present disclosure provides a series battery pack capacity on-line monitoring and charging/discharging dual-state equalization circuit and method. The series battery pack capacity on-line monitoring and charging/discharging dual-state equalization circuit includes a storage battery pack, a gating switch array, a power dissipation loop, a polarity matching switch array, an isolated DC/DC converter, a bipolar differential battery voltage measurement circuit, a micro controller unit, a charging/discharging current detection loop, and a switch circuit unit. According to the present disclosure, each series battery of the storage battery pack can be separately charged and discharged by controlling the gating switch array, the storage battery pack capacity can be detected on line, bypass discharge and parallel supplement can be carried out on the series batteries with high or low relative state of charge in two states of charging and discharging, and the power of each series battery can be efficiently equalized.