A data writing method for writing updated data from a host into a memory module is provided. Herein, some physical units of the memory module are gotten to be global random physical units for storing data from the host. The method includes determining whether the updated data is sequential data and determining whether a logical page corresponding to the updated data is a start logical page. The method further includes getting a blank physical unit from the physical units as a new global random physical unit and writing the updated data into the new global random physical unit when the updated data is the sequential data and the logical page corresponding to the updated data is the start logical page. Accordingly, the method can write updated data belonging to the same logical unit into the same physical unit, thereby shortening the time for executing write commands.