A data storage device with a FLASH memory accessed via multiple channels and a FLASH memory control method are disclosed. The control method includes dividing a plurality of blocks of a FLASH memory into groups to be accessed by a plurality of channels separately, each block comprising a plurality of pages; allocating a random access memory to provide a first set of cache spaces for the different ones of the plurality of channels; separating write data issued from a host to correspond to the plurality of channels; and after data arrangement in the first set of cache spaces for every channel is completed, writing data arranged in the first set of cache spaces for every channel to the FLASH memory via the plurality of channels. The control method further includes allocating the random access memory to provide a second set of cache spaces; and using the second set of cache spaces to perform data arrangement for the write data issued from the host when writing the data arranged in the first set of cache spaces for every channel to the FLASH memory.