The invention discloses an indexing device for a data storage system which comprises a plurality of data storage devices. The indexing device generates an I/O descriptor index number according to a target data storage device, where the I/O descriptor index number corresponds to a device ID number and a queued command tag number. After receiving from the target data storage device an information packet containing the queued command tag number and a second connection request data frame including the device ID number, the indexing device can calculate the I/O descriptor index number according to the device ID number and the queued command tag number.