A method for determining a parity check matrix utilized in a flash memory system is disclosed. The parity check matrix comprises M×N blocks. The method includes generating a first set of candidate blocks as candidates of a first set of blocks of the M×N blocks; calculating a plurality of first estimated results corresponding to the first set of candidate blocks; determining content of a first block of the M×N blocks according to a best result of the first estimated results; generating a second set of candidate blocks as candidates of a second set of blocks of the M×N blocks; calculating a plurality of second estimated results corresponding to the second set of candidate blocks by considering the content of the first block; determining content a second block of the M×N blocks according to the second estimated results.