This application provides a method and device for estimating a SOC of a battery pack, and a battery management system. The battery pack includes a first cell without a plateau and a second cell with a plateau. At least one first cell is serially connected to the second cell. The method includes: determining a capacity variation of the battery pack based on a SOC variation of the first cell in comparison with an initial SOC of the first cell, and based on a nominal capacity of the first cell; obtaining an equalization capacity of the first cell and an equalization capacity of the second cell; and estimating a SOC of the second cell, and determining the SOC of the battery pack based on the SOC of the second cell.