A method for indicating storage capacity is applied in a storage device. The storage device includes a main storage unit, a capacity indicator, a driver, and a secondary storage unit. The second storage unit stores a preset storage capacity value and a drive table recording relationship between storage capacity difference ranges and rotation angles of the driver. The method includes: periodically obtaining a storage capacity value from a computer connected to the storage device; computing a difference between the obtained storage capacity value and the preset storage capacity value; determining the storage capacity difference range the difference falls within and the rotation angle the determined storage capacity difference range corresponds to in the drive table; and controlling the driver to rotate the determined rotation angle to drive the capacity indicator to indicate the obtained storage capacity value. A related storage device is also provided.