1. A flash memory storage system, comprising: a controller; a connector, electrically connected to the controller; a cache 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 wherein when the controller receives a flush command from the host, the controller temporarily stores data stored in the cache memory into the SLC NAND flash memory, wherein when rebooting, the controller determines whether the SLC NAND flash memory stores the data, and if the SLC NAND flash memory stores the data, the controller reloads the data temporarily stored in the SLC NAND flash memory to the cache memory.