When there is mainlobe interference, the adaptive beamforming based on eigenprojection preprocessing (EPB) can effectively suppress the mainlobe interference and sidelobe interference. However, when the power of mainlobe interference becomes stronger, the ability of the algorithm to suppress sidelobe interference is greatly decreased, and the sidelobe interference fails to be suppressed effectively. In accordance with EPB, an algorithm based on eigenprojection preprocessing and augment of null depth is proposed in this paper. Firstly, the eigenprojected data is projected on the interference signal subspace which removes the main lobe interference. Secondly, the data after projection is weighted appropriately and then added to the data after eigenprojection preprocessing to enhance the power of the sidelobe interference signal. Thirdly, the output of the modified weight vector can be calculated by the new covariance matrix which is formed by the sampled data after interference enhancement. Fourthly, the output of the adaptive beamforming can be calculated by the modified weight vector, and the signal to interference plus noise ratio of the algorithm is analyzed briefly. The simulation results show that the proposed algorithm in this paper can form deeper nulls in the direction of sidelobe interference, and obtain still greater sidelobe suppression effect in the case of higher power of the mainlobe interference.