1. A flash memory storage system, comprising: a controller; a connector, electrically connected to the controller; a flash memory, electrically connected to the controller; a single level cell (SLC) NAND flash memory, electrically connected to the controller; and a multi level cell (MLC) NAND flash memory, electrically connected to the controller, wherein when the controller receives data to be written into the MLC NAND flash memory from a host via the connector, the controller temporarily stores the data into the cache memory, and then writes the data into the MLC NAND flash memory from the cache memory, and the controller temporarily stores data stored in the cache memory into the SLC NAND flash memory.