A precoding method is provided. The precoding method includes computing a relaxed beamforming matrix according to multiple desired channel correlation matrices and multiple interfering channel correlation matrices; computing an approximated beamforming matrix according to the relaxed beamforming matrix; computing multiple degradations corresponding to the data streams according to multiple relaxed beamforming vectors within the relaxed beamforming matrix and multiple approximated beamforming vectors within the approximated beamforming matrix; selecting a selected data stream index according to the degradations; decomposing a selected relaxed beamforming vector corresponding to the selected data stream index into a first vector and a second vector; and updating the approximated beamforming matrix according to the first vector and augmenting the approximated beamforming matrix according to the second vector, to obtain an updated-and-augmented beamforming matrix.