A data reading method for a flash memory module is provided. The method includes applying a bit-data-read voltage to get read data from memory cells of the flash memory module. The method also includes setting a minus-adjustment-bit-data-read voltage and a plus-adjustment-bit-data-read voltage corresponding to the bit-data-read voltage based on an error-distribution estimated value and applying the minus-adjustment-bit-data-read voltage and the plus-adjustment-bit-data-read voltage to obtain soft values corresponding to the read data from the memory cells. The method further includes calculating a soft-information estimated value corresponding to each bit of the read data according to the soft-values. Accordingly, the method can effectively obtain soft information.