Abstract: Standard Capon beamformer has serious distortion in the small snapshot or steering vectors mismatched or high SNR. In order to improve the robustness of beam, a robust Capon Beamforming with Orthomodular Constraints algorithm (OCCB) is proposed, through the second derivative get the accurate mathematical expression of weights. The algorithm restrains the orthogonality of array weight and noise subspace in the base of Capon, achieving diagonal loading of noise eigenvalue, decreasing spread level of noise eigenvalue. The influence of partly diagonal loading to the desired signal, interference and noise is analyzed. In the small snapshot or steering vectors mismatched or high SNR, this algorithm can make beam get lower sidelobe and more accurate mainlobe direction. Meanwhile, the interference can be better suppressed. Simulation results confirm the effectiveness of the proposed algorithm.