The codec includes an encoding/decoding operation module and a basic matrix storage module. In the stored basic matrix Hb, for all girths with length of 4, any column element of i, j, k or l constituting the girths in anti-clockwise or clockwise always satisfies inequality: (i−j+k−1) mod z≠0, wherein z is the extension factor. When generating the basic matrix, firstly the number of rows M, number of columns N, and weight vectors of the rows and columns are determined, an irregularly original basic matrix is constructed; then the position of ‘1’ is filled by a value chosen from set {0, 1, 2, . . . , z−1} to obtain the basic matrix Hb. The basic matrix Hb obtained by storing constitutes the desired encoder/decoder. The encoder/decoder according to the present invention can effectively eliminate error-floor phenomenon of LDPC codes and accelerate the falling speed of BER curve.