Abstract: This paper proposed a physical layer secrecy coding algorithm using multi-antenna channel characteristics projection in consideration of the existing secrecy coding,s problems, such as channel condition dependence and random sharing. In channel reciprocal Time-Division-Duplex (TDD) system, multi-antenna transmitter estimates the authorized channel characteristics according to the training symbols transmitted by the single-antenna receiver, and then generates the pair of projected vectors through multi-antenna channel characteristics projection.While transmitting weighted vector is randomly selected from the pair of projected vectors symbol-by-symbol, the Hamming distances of wiretapper,s demodulated codewords are radomly confused so that it is hard to decode normally and secure transmission is qualified. Simulation results show that the scheme makes the wiretapper,s BER near approximately 0.5 and the authorized receiver,s BER a level lower than the existing methods of multi-antenna physical layer secure transmission.