A method and device for the baseband process of space-time / space-frequency / spatial diversity transmitter in the SC-FDMA system, the device is characterized in that: 1) an encoding means is connected to the output end of the M-point DFT module; 2) a space-time encoding means, for encoding the corresponding symbol of at least two adjacent symbol sets, and outputting at least two data sets; each sub-carrier mapping means maps respectively each data set to the corresponding antenna, so that the mapped data satisfy the requirements: The symbols mapped on each antenna keep the same sequence as the M symbols outputted by the M-point DFT module; the mapped symbols keep the same sub-carrier interval; the M data of each mapped SC-FDMA symbol are the M outputted data of the DFT module, or the inverse of the M outputted data, or the complex conjugate of the M outputted data, or the inverse complex conjugate of the M outputted data. The present invention implements effective space diversity, reduces PAPR, and makes frequency domain equalization feasible.