The security of traditional wireless communication is mainly dependent on the upper layer encryption mechanism

which cannot protect the physical layer modulation mode and the modulation information security.This paper proposes a physical layer encryption modulation method for the orthogonal frequency division multiplexing (OFDM) system.In this method

the phase random signal is used to detect and extract the equivalent channel phase information

then the 3D (Three-Dimensional) constellation is modulated in the physical layer

followed by the receiver's decryption and demodulation.The scheme makes full use of the wireless channel phase response of randomness

reciprocity and location sensibility

and this characteristic is used to enhance the safety of physical layer signal.Theoretical analysis and computer simulation results show that the algorithm is secure

and compared with the traditional QPSK (Quadrature Phase Shift Keying) modulation

the BER (Bit Error Rate) of the proposed algorithm improves about 2.5dB.This paper focuses on the physical layer security

which aims at providing a new security for the physical signal layer

and has a wide application prospect in the future 5G communications and military security communications.