Provided is an encoding method and device and a decoding method and device for structured LDPC. The encoding method includes: determining a base matrix used for encoding and performing an LDPC encoding operation on a source information bit sequence according to the base matrix and an expansion factor Z corresponding to the base matrix to obtain a codeword sequence, where Z is a positive integer. The base matrix includes multiple submatrices and the submatrices include an upper-left submatrix Hb1 and an upper-left submatrix Hb2, and the upper-left submatrix Hb1 is an upper-left submatrix of the upper-left submatrix Hb2.