A data storage device and a FLASH memory control method with a cache space. The FLASH memory control method includes the following steps: using a plurality of channels to access a FLASH memory, wherein the FLASH memory has a plurality of blocks each with a plurality of pages, and the blocks are grouped to be accessed by the different channels; allocating a random access memory to provide a cache space, the cache space having a plurality of cache areas caching write data for the different channels, respectively; distributing the data issued from a host to correspond to the different channels; and reusing a latest-updated cache area of the cache space to cache write data when a logical address requested to be written with data is identical to a logical address that the latest-updated cache area corresponds to.