This paper proposes a novel mainlobe interference suppression method based on eigen-projection preprocessing and beam pattern reshaping. In view of the eigen-beam, we can easily determine the existence of the multiple mainlobe interferences, and then construct the eigen-projection matrix to ρ reprocess the echo data. Meanwhile, a convex programming model is used to reshape the mainlobe beam pattern, which yields the adaptive weight vector. Consequently, the output of adaptive beamforming is obtained by the processed echo data and the adaptive weight vector. This method can enjoy low sidelobe level and well-maintained mainbeam of the beampattern. Synthetic performance analysis and simulation results have demonstrated the superior performance of the proposed method.