A block management method for managing a plurality of physical blocks is provided. The method includes grouping the physical blocks into a plurality of physical units, grouping a portion of the physical units into a data area and a spare area, configuring a plurality of logical units, and grouping the logical units into a plurality of logical unit groups and configuring another portion of the physical units as a plurality of global random physical units corresponding to the logical unit groups, wherein each of the global random physical units corresponds to one of the logical unit groups. The method further includes getting the physical units from the spare area as global random substitute physical units of the global random physical units. Accordingly, the method can store data in the global random physical units or the global random substitute physical units, thereby reducing the time for executing a host write command.