The invention discloses a Raptor Codes encoding / decoding method suitable for medium / short code lengths of an additive white Gaussian noise channel. According to the method disclosed by the invention, the degree distribution of the inner codes, i.e. LT (Linear-Time) Codes, of the Raptor Codes which are more suitable for the medium / short code lengths is proposed; and the improved PEG (Progressive Edge-Growth) algorithm is used for encoding the inner codes, i.e. the LT codes, of the Raptor Codes, wherein the encoding manner consists of two PEG encoding manners, i.e. independently carrying out PEG composition on the LT Codes and carrying out the PEG composition on the LT Codes through combining with outer codes, i.e. LDPC (Low Density Parity Check) Codes, thus the problem that excessive shortloops appear in a Tanner graph, caused by randomly selecting information packets in an LT encoding stage, is solved. With the adoption of the method disclosed by the invention, the effectiveness for the transfer of soft information when the BP (Background Processing) iterative decoding is carried out is therefore enhanced, the performance better than that of a random composition manner is obtained, and the more reliable transmission for the rateless codes with the medium / short code lengths is realized.