The writing method includes: grouping logical erasing units into a first region and an second region; determining if a first logical erasing unit which a host system intends to write belongs to the first region or the second region; if the first logical erasing unit belongs to the first region, writing data to a spare physical programming unit, wherein the physical erasing unit to which the spare physical programming belongs further stores data belonging to another logical erasing unit; if the first logical erasing unit belongs to the second region, writing data to a physical erasing unit in which all the valid data belong to the first logical erasing unit. Accordingly, a speed of sequential writing is guaranteed to be greater than a target value.