Abstract: Diagonal loading has been a popular approach to improve the robustness of beamforming algorithm. On the basis of diagonal loading, the eigenvalue decomposition of the covariance matrix of receiving data can be calculated. Then the range of diagonal loading value is given according to the matrix inversion theorem. The optimum equation and the weight vector of space-time beamforming algorithm based on diagonal loading is then proposed. The proposed method improves the robustness of space-time adaptive beamforming algorithm effectively. Results of computer simulation verify the efficiency of the method.