The method includes following steps: 1) the system is initialized; the numbers of cyclic prefix inserted into OFDM system are selected by actual measurement of multi path time delay; according to thecyclic prefix, the frequency inserted into by the frequency domain pilot frequency is determined ; 2) SINAD threshold of each kind modulation mode like BPSK, QPSK, 16QAM, 64QAM is set by high layer protocol, the SINAD value obtained from step 1 is compared with the threshold corresponding to each modulation mode; 3) Doppler shift is estimated out, it is decided that the product of Doppler shift andframe length is which range; 4) for first sub-carrier in each group, the modulation mode is presented by using GRAY CODE, the transmission of signal uses 4 bit; 5) the transmitting end assigns the mo dulation mode to next frame data according to received TMC instruction.