An encoding method, encoding device, decoding method and decoding device for low density generator matrix codes (LDGC) are disclosed. Wherein, the encoding method comprises: construct an LDGC mother code set using P LDGC with code rate R0 and different code lengths, wherein the LDGC mother code set has a uniform basic matrix Gbuniform wherein, R 0 = k b n b , kb denotes the number of rows and nb denotes the number of columns in the basic matrix; obtain length L of an intermediate variant according to length K of an information bit sequence to be encoded in the LDGC mother code set; modify and expand the basic matrix to obtain a generator matrix Gldgc using the length L of the intermediate variant and the number kb of rows in the basic matrix; and encode the information bit sequence to be encoded using a matrix Gldgc (1:L,1:N+L−K) composed of L rows and the front N+L−K columns of the generator matrix, wherein N denotes the length of the encoded information.