A method applied into a controller of a Bluetooth device includes: providing a first flag and a second flag which are configured as a default value respectively; changing the first flag from the default value when the controller successfully receives the particular packet transmitted from the audio gateway; changing the second flag from the default value when the controller successfully receives an acknowledgement from a secondary device wherein a reception of the acknowledgement indicates that the secondary device successfully receives the particular packet; and transmitting an acknowledgement of a particular packet to an audio gateway when detecting that either the first flag or the second flag changes and becomes different from the default value.