The invention provides a system LT code compiling method based on the superposition degree. The method comprises the following compiling steps: conducting initialization, defining input nodes S, middle nodes I and redundancy check nodes R, constructing a BP translatability matrix in advance, and utilizing S=I G to generate the I; generating the R by the I in a distribution mode through the superposition degree; using the S and the R as output nodes to be sequentially sent out. SPDD degree distribution is designed and optimized for the middle nodes of system coding, a DD doping level component is arranged in a superposition mode on the basis of WRSD degree distribution, the full coverage probability on information nodes is effectively ensured, and meanwhile the method has the advantages of ensuring linear code compiling complexity and partial recovery of the middle nodes and theoretically proving the asymptotic performance of the SPDD.