The adaptive monopulse technology is a common method for phased array radar to achieve target detection and angle estimation. In order to solve the problem that the performance of the traditional adaptive beamforming algorithm is reduced and the monopulse angle estimation results are greatly deviated in the complex environment where the mainlobe and sidelobe interference coexists, a two-stage mainlobe and sidelobe interference suppression method of adaptive monopulse is designed. The first stage of adaptive processing uses the improved GSC structure to complete the suppression of sidelobe interference, and at the same time, the blocking matrix is constructed to protect the target signal and mainlobe interference from being cancelled in the suppression process of sidelobe interference, so as to improve the mainlobe maintenance effect remarkably. The second stage of adaptive processing uses the sum-difference four-channel mainlobe interference suppression to complete the directional suppression of mainlobe interference,that is, while the mainlobe interference in the elevation or azimuth direction is adaptively suppressed, the non-adaptive array pattern of the azimuth or elevation direction is maintained, and then the monopulse ratio maintains undistorted relative to the quiescent monopulse ratio. The simulation results demonstrate that the proposed method can effectively suppress the mainlobe and sidelobe interferences without distortion of monopulse ratio at the same time.