Flash memory system and flash memory device thereof

A flash memory system, comprising:

• a host, fetching data;
• a flash memory device, coupled to the host, and accessed by the host for data, wherein the flash memory device comprises:
• an NAND flash memory, comprising:
• an NAND flash memory array, comprising a plurality of pages, wherein each page comprises a plurality of sub-pages, each sub-page has a sub-page length;
• a cache memory, the cache memory is composed of a plurality of sub caches and the plurality of sub caches corresponds to different pages of the NAND flash memory array;
• and
• a page buffer, the page buffer is composed of a plurality of sub-page buffers and the plurality of sub-page buffers corresponds to different pages of the NAND flash memory array, wherein each of the plurality of sub-pages of the same sub-page offset in the page buffer and in the cache memory come from different pages of the NAND flash memory array;
• and
• a control circuit, coupled to the host and the NAND flash memory, performing an access operation in units of one sub-page.