A method for managing a multi-channel memory device includes at least following steps: when the multi-channel memory device is controlled to operate in an M-channel mode, reserving a partial memory space across N memory channels of the multi-channel memory device, where the reserved partial memory space is not used under the M-channel mode, M and N are positive integers, and M is smaller than N; and when the multi-channel memory device is controlled to switch from the M-channel mode to an N-channel mode, accessing data in the reserved partial memory space across the N memory channels used under the N-channel mode. The method for managing a multi-channel memory device can improve switch response time.