The invention relates to a base matrix, a encoder/decoder of Low Density Parity Check (LDPC) codes and a generation method thereof. The encoder/decoder is determined uniquely by the parity check matrix of the LDPC codes. With different code sizes, said parity check matrix can be obtained by expanding different base matrixes, and also can be obtained by expanding a unique base matrix after correction. Elements of said base matrix must meet an inequation in which the girth value is up to the standard, e.g. when girth ≧6, for any element i, j, k, l in the matrix which forms the short loop having a length of 4 in anticlockwise, there are always mod(i−j+k−l, z) unequal to 0. By this invention, the girth of the constructed base matrix is made as large as possible, the amount of the shortest loops is as few as possible when it is the same girth, thus the curve of bit error ratio (BER) is dropped quickly, the error floor of the LDPC codes is eliminated effectively, and the optimal performance of the LDPC codes is obtained.