A novel endtoend data transmission scheme based on OFDM mechanism with good robustness against the RPE-LTP vocoder in GSM voice channel is proposed. According to the character of RPE-LTP vocoder, data are modulated to orthogonal lowfrequency sinusoidal carrier. In OFDM, the modulation is done by IFFT and the modulated signals are conformed to the time and frequency characters of RPELTP vocoder. The results of simulation show that the transmission bit rate of the proposed scheme reaches 2.4 kb/s，and the bit error rate of the scheme is lower than 0.3 %. This scheme provides a new approach for realtime secure voice data transmission over GSM system.