Examples of an equalization circuit, a battery pack, and an energy storage system are described. In one example, an equalization circuit is configured to: when a difference between a state of charge (SOC) of a battery pack and an SOC or SOCs of any one or more other battery packs exceeds an equalization threshold, by turning on or off switch transistors of a bridge resonant circuit and a bridge circuit, receive, through an equalization bus, discharge power of the one or more other battery packs, and input charge power to the battery pack; or receive discharge power output by the battery pack, and output, to the equalization bus, charge power for inputting to the one or more other battery packs, so that uniformity between SOCs of the battery packs can be flexibly adjusted.