The present invention discloses a coding method, a coding device, a decoding method and a decoding device for low density generator matrix codes. Wherein the coding method includes: constructing the mother code set of the low density generator matrix code using P low density generator matrix codes of different code lengths at a code rate R0, wherein the mother code set of the low density generator matrix code has a uniform basic matrix Gbuniform, R 0 = k b n b , where kb is the row number of the basic matrix and nb is the column number of the basic matrix; obtaining the length L of an intermediate variable according to the relationship between the length K of the information bit sequence to be coded in the mother code set of the low density generator matrix code and the length L of the intermediate variable; obtaining an expansion factor zk for processing the basic matrix using the length L of the intermediate variable and the row number kb of the basic matrix, and obtaining a generator matrix Gldgc, by modifying and expanding the basic matrix using the expansion factor zk; and coding the information bit sequence to be coded using a matrix Gldgc(1:L,1:N+L−K) comprised of L rows and the first N+L−K columns of the generator matrix.