Since adaptive beamforming suffers from output performance degradation in the presence of interference nonstationarity or signal steering vector mismatch, a new robust null broadening technique is proposed. The proposed method can pre-process the array received data by projection transform technique to get a new covariance matrix, so the interference incident angle is extended and the null is broadened. Then, magnitude response constraints are applied on general location of the desired signal. The proposed method forms a stable magnitude response in the constraint region against the steering vector mismatch. The proposed method can be solved by converted it into a Semi-definite Relaxation programming. Simulation results show that the proposed method can effectively broaden the beam null width and enhance the null depth, which can suppress strong interference with fast motion and has high ability against steering vector mismatch. The robustness of beamformerhas been significantly improved due to the proposed method in complex situations.