In order to ensure the safe transmission of the time reversal-orthogonal frequency division multiplex （TR-OFDM） system， this paper proposes a secure transmission algorithm based on quaternion encryption. The scheme is mainly divided into three steps. In the first step， the transmitter and the legitimate receiver use the estimated channel to obtain the quaternion required in the process of encrypted transmission， that is， the key. In the second step， the transmitting end takes the bit sequence to be transmitted three-dimensional mapping， then uses the quaternion to rotate and encrypt the three-dimensional constellation points， and finally modulates them into OFDM symbols and transmits them after time inversion processing. In the third step， the legitimate user uses the quaternion to decrypt and demodulate to obtain the transmitted data. The eavesdropper cannot obtain the transmission information because they do not know the key. Therefore， the proposed scheme ensures the security of system data transmission. The simulation results show that the proposed algorithm can keep the bit error rate of eavesdropping users at about 0.5. Under the same SNR， legal users can achieve a lower bit error rate than traditional two-dimensional modulation. Compared with the artificial noise scheme， the proposed algorithm will not affect the bit error rate of legitimate users.