Network Coding - Aiming at the problem of security transmission in the space-terrestrial integrated networks, this paper proposes a physical layer secure transmission architecture based on concatenated LT and LDPC Codes. The outer code LT code adopts real-time random sampling coding, which can form a complex random interconnection structure and fully expand the randomness. The inner code LDPC code generates different codes through the random change of cyclic shift vector, Reduce the node interconnection deterioration caused by randomization on soft decision decoding, and obtain weak randomness high-performance error correction coding through storage optimized high-performance check matrix combined with soft decision decoding. The analysis and simulation results show that the proposed security transmission method not only improves the security of transmission, but also maintains the high transmission efficiency. Therefore it can be applied to the field of secure communication.