A data writing method for moving data in a plurality of flash memory modules during a write command of a host system is executed is provided, wherein each of the flash memory modules has a plurality of physical blocks. The present data writing method includes transferring first data received from the host system to one of the flash memory modules and writing the first data into the physical blocks of the flash memory module according to the write command. The present data writing method also includes moving at least one second data in the physical blocks of another one of the flash memory modules during the first data is written. Thereby, when the host system is about to write data into the other flash memory module, the time for executing the write command is effectively reduced.