The present invention discloses a hybrid density memory storage device configured to store data responsive to a host and a file system thereof. The hybrid density memory storage device includes a non-volatile memory, a hot data buffer and a control unit. The non-volatile memory includes a high density storage space and a low density storage space. The control unit is coupled between the host, the non-volatile memory, and the hot data buffer. The control unit has a hot list used for recording a plurality of logical locations of hot data, and the control unit is capable of accessing data in/out the hot data buffer in accordance with the hot list.