A method for writing update data into a flash memory module to replace original data is provided. The flash memory module includes at least one block including a plurality of sectors. Each of the sectors records a flag. The data storing method includes: getting a first sector, wherein all the bits of the flag of the first sector are the second storage status; writing the update data into the first sector, and programming at least one bit as the first storage status and at least one bit as the second storage status in the flag of the first sector; identifying a second sector storing original data, wherein at least one bit of the flag of the second sector is first storage status and at least one other bit is second storage status; programming all the bits of the second sector as the first storage status.