A method for capacity expansion of an energy storage system and an energy storage system are provided. The method is applied to a controller in the energy storage system. The controller acquires a value of a characteristic parameter of a to-be-added second energy storage device as a target characteristic value. Then, the controller controls a charge/discharge power converter to charge or discharge first energy storage devices until a proximity of a value of a characteristic parameter of at least one first energy storage device to the target characteristic value is less than a preset proximity. Finally, a signal indicating that the second energy storage device is allowed to be connected into the energy storage system is generated and outputted to notify an operator.