A decoding method for a memory storage device including a rewritable non-volatile memory module is provided according to an exemplary embodiment of the invention. The method includes: reading at least one memory cells by using at least one read voltage level to obtain a codeword; performing a parity check operation on the codeword by an error checking and correcting circuit to generate a syndrome sum corresponding to the codeword; and dynamically adjusting a first parameter used by the error checking and correcting circuit in a first decoding operation based on whether the syndrome sum is less than a first threshold value and performing the first decoding operation on the codeword by the error checking and correcting circuit by using the first parameter.